The Production of Thyroid Hormone Part Two
The thyroid has far reaching effects on health from fetal development to adulthood. Successful treatment of thyroid disorders requires a solid understanding of the production of thyroid hormone and how this relates to thyroid disorders and their treatments. In this chapter, we take a deep dive into the molecular biology and pathophysiology of the normal thyroid gland at the cellular and biochemical level. In addition, we will explore thyroid disease pathophysiology and thyroid drug treatment from this same point of view.
Header Image: Thyroid follicles microscopic view courtesy of wikimedia commons and Grays Anatomy.
What is Thyroid Hormone ?
The main job of the thyroid gland is to produce and secrete thyroid hormone into the blood stream in a ratio of 80% T4 and 20% T3. This thyroid hormone is transported in the blood stream to a trillion cells in our body, the periphery. Once the T4 thyroid hormone reached the cell level in the periphery, it must be converted to T3, its active form. This conversion is done by the the D1 Diodinase Enzyme. T4 is a pro-hormone, having four iodine atoms attached to two tyrosine rings coupled together. The De-Iodinase Enzyme is intracellular, and removes an iodine from T4, thus converting T4 into T3. This intracellular T3 is the active form of thyroid hormone which increases intracellular energy and serves as a “major endocrine controllers of metabolic rate” as we will discuss below.
The other two De-Iodinase enzymes are D2 which is located in the pituitary, and D3 which converts T4 to inactive reverse T3.
Chemical Structure of thyroid hormone: Above left image is T4 with four iodine atoms, and above right image is T3 with three Iodine atoms. Courtesy of de Castro, Guilherme Vieira, et al. “Synthesis of Analogues of Thyroid Hormones: Nuclear Receptor Modulators.” Orbital: The Electronic Journal of Chemistry 7.3 (2015): 282-291.(57)
The Major Controller of Metabolic Rate
In 2014, Dr Carla Portulano discusses the importance of thyroid function for overall human health, writing this importance is difficult to overstate:
The significance of the thyroid gland for human health is difficult to overstate, given the wide-ranging effects of the thyroid hormones on prenatal and early development as well as on intermediary metabolism at all stages of life. (32)
Increases Size and Number of Mitochondria
Thyroid hormones have been described as “the major endocrine controllers of metabolic rate”. An increase in thyroid hormone level will increase metabolic rate, primarily via effects on mitochondria, stimulating increased numbers and size of mitochondria (mitochondriogenesis) as well as respiratory chain components within the mitochondria. Left image: Electron Microscopy of mitochoindria courtesy of wikimedia commons.
In 2008, Dr. Mary Ellen Harper discusses the effect of thyroid hormone on mitochondria, writing:
thyroid hormones stimulate mitochondriogenesis and thereby augment cellular oxidative capacity. Thyroid hormones induce substantial modifications in mitochondrial inner membrane protein and lipid compositions. Results are consistent with the idea that thyroid hormones activate the uncoupling of oxidative phosphorylation through various mechanisms involving inner membrane proteins and lipids. Increased uncoupling appears to be responsible for some of the hypermetabolic effects of thyroid hormones. (20)
In 2022, Dr Federica Cioffi discusses how thyroid hormones increase specific components of the mitochondrial respiratory chain, writing:
almost all components of the respiratory chain [within mitochondria] are directly or indirectly affected by iodothyronines [thyroid hormones]. In some cases, the actions result in an activation of specific biochemical pathways while in others the effects would result into an increase in mRNA or protein levels of specific components of respiratory chain. (21)
Above image: light microsopy of thyroid follicles courtesy of wikimedia commons.
The Thyrocyte and Follicular Lumen:
The workhorse of the thyroid gland is the thyrocyte, a cuboidal cell arranged in a circular array surrounding the follicles, the round “storage tanks” filled pink staining colloid, also called thyroglobulin. Microscopically, the thyroid gland demonstrates a pattern with thousands of small spherical follicles serving as storage chambers for colloid, thyroglobulin, the precursor storage protein converted to thyroid hormone by adding iodine molecules in a process called organification.
The NIS Sodium Iodide Symporter
The thyrocytes are triangular shaped and have a specific orientation. The apex is nearest the lumen of the follicle and contains the villous apical membrane which the important work of organification. The basal membrane of the thyrocyte contains the NIS, the sodium iodine symporter, the active transport mechanism which takes up iodide from the blood stream and concentrates iodide 20-50 times that of plasma. (76)
Five Steps of Thyroid Hormone Synthesis
In 2022, Dr. Muhammad Shahid discussed the five steps of thyroid synthesis.(1)
First Step: Synthesis of Thyroglobulin:
Thyrocytes in the thyroid follicles produce a protein called thyroglobulin (TG), the precursor to thyroid hormone. TG is secreted by exocytosis and stored in the follicle. Initially, thyroglobulin does not contain iodine. However, over time, the thyrocyte attaches iodine to the thyroglogulin in a process called organification.
Step Two: Uptake of Iodide, the Sodium Iodide Symporter:
TSH stimulation causes increased activity of the Sodium Iodide Symporter (NIS), the active transport of iodide into the thyrocyte. This is a pretein imbedded within the basolateral membrane of the thyrocyte. The NIS actively pumps iodide into the cell, maintaining a concentration 20 to 50 times higher than that of the blood stream. In Grave’s hyperthyroidism, this ratio is increased to greater than 100 times higher, due to massive TSH receptor stimulation by TSI and TRAb antibodies. Even though the serum TSH will be suppressed to very low levels in Graves’ Disease, the Graves’ disease TSH receptor antibodies serve in their place producing massive stimulation to make thyroid hormone.(34)(89-97)
Concentrating Lithium and Bromine
The NIS in the thyrocyte basal membrane will also concentrate other ions such as lithium and bromine. Lithium is discussed in its own chapter. If bromine is toxic to the thyroid gland , and if concentrated in the thyroid, this may interfere with iodine uptake and concentration. If severe, this may lead to iodine deficiency and hypothyroidism.(58)
The Iodide is pumped into the thyrocyte by the NIS at the basolateral membrane and diffuses through the thyrocyte towards the apical membrane where it is pumped by the Pendrin transporter into the follicular lumen. Note: the role of Pendrin is still matter for debate. (1-4)(15)
Step Three : Iodination of Thyroglobulin:
Thyroglobulin provides the polypeptide backbone for synthesis and storage of thyroid hormone within the follicle. The thyrocyte manufactures thyroglobulin within the endoplasmic reticulum, and uses exocytosis to secretes the thyroglobulin into the follicular lumen. This thyroglobulin within the follicular lumen is called Colloid. Thyroglobulin becomes thyroid hormone by the addition of iodine in a process called iodination, or organification, carried out by the Thyroperoxidase (TPO) enzyme, a heme containing molecule with a porphyrin ring structure similar to hemoglobin.
TPO is secreted into the follicular lumen at the thyrocyte apex villous membrane, and has three functions:
1) oxidation of iodide(I-) to iodine (I2)
2) organification of iodine (attachment of iodine(I2) to thyroglobulin)
3) coupling together of two Tyrosine residues
Once iodide (I-) is oxidized by TPO to molecular iodine (I2), the molecular iodine then combines with ring-like Tyrosine residues on the thyroglobulin to form MIT and DIT (mono-iodo-tyrosine and di-iodotryrosine).
Oxidation of Iodide to Iodine: TPO uses hydrogen peroxide to oxidize iodide (I-) to iodine (I2). The hydrogen peroxide is generated by NADPH oxidases, dual oxidases 1 and 2 (DUOX1 and DUOX2) located in the villous apex of the thyrocyte. Iodide, a negative ion, is oxidized to molecular iodine (I2) by the TPO Thyroperoxidase enzyme using hydrogen peroxide as a substrate. This takes place inside the follicles on the micro-villous surface of the apical membrane of the thyrocytes, the same location as hydrogen peroxide produced by the DUOX enzyme system.
Organification: Iodine(I2) spontaneously attaches to Tyrosine residues of Thyroglobulin protein, generating monoiodotyrosine (MIT, one iodine) and diiodotyrosine (DIT, two iodines)
Coupling reaction: Iodinated Tyrosine residues are coupled by TPO making triiodothyronine (T3) and tetraiodothyronine (T4). T4 is made from 2 DIT’s coupled together. T3 is made from MIT and DIT coupled together.
Step 4: Storage: Thyroid hormones (T3 and T4) are stored in the follicular lumen bound to thyroglobulin as colloid, a combination of free and iodinated thyroglobulin.
Step 5: Release of thyroid hormone into circulation: Iodinated thyroglobulin is taken up at the apex of the thyrocytes within vesicles via endocytosis, and travels back towards the basal membrane. While in transit, these vesicles are fused with lysosomes containing acid and proteolytic enzymes which digest the thyroglobulin, freeing the T3 and T4, subsequently released at the basolateral membrane into the capillary blood stream in ratio of 80% T4 and 20% T3. Any extra iodine is salvaged and returned to the intracellular iodine pool.(1-4)
Hashimoto’s Thyroiditis:
In Hashimotos’ Thyroiditis, blood tests show elevated antibodies to the TPO (Thyroperoxidase) enzyme and to thyroglobulin. Both are in close proximity to the hydrogen peroxide generating system called DUOX (dual oxidase system) at the apical villous membrane. One hypothesis is that damage caused by excess hydrogen peroxide to these proteins creates antigenicity and autoimmunity. Secondly, in Hashimoto’s Thyroiditis, microscopic examination of the thyroid shows lymphocytic infiltration within the thyroid gland. Thus the auto-immunity is represented by both antibody (B cell) and cellular immunity (T cell).
Immune Complexes Deposited Along Basement Membrane
In both Graves’ and Hashimoto’s Disease, electron microscopy may show electron dense deposits along the basement membrane of the follicular cells which are immune complexes when visualized with immunoflourescent staining. This could represent antibodies to the NIS, which is discussed in 2020 by Dr. Anna-Marie Eleftheriadou. When present, this may cause dysfunction of the NIS in these disease entities. (34) (59-61)
Immunoflourescent staining in Hashimoto’s with high titers of TPO antibodies shows intense staining of the cytoplasm of thyrocytes without staining of colloid in follicles. This is the location where TPO is manufactured. In cases with high thyroglobulin antibody titers, there is diffuse staining of colloid within the follicles. This is where the thyroglobulin is stored. (73)
Organification Defect in Hashimoto’s – Perchlorate Discharge
Hashimotos’ disease results in organification defect, a decreased ability to organify iodine. This is called “inability to organify iodine”, and explains the low iodine content of the thyroid gland in Hashimoto’s patients, and the rapid washout of iodine with the perchlorate discharge test. Perchlorate is a competitive inhibitor of the NIS, sodium iodide symporter, and blocks iodide uptake, so any non-organified iodine remaining in the thyroid gland will “wash out”. The test is useful for identifying an organification defect. (101-108)
Perchlorate Washout Test
In patients with lymphocytic thyroidits, also called Hashimotos’ Thyroiditis, their perchlorate discharge test will show a positive test, meaning a large amount of iodine will wash out and not be retained by the thyroid. This indicates an organification defect. However, in normal patients, the iodide is oxidized to iodine by TPO and H202, and then organified, i.e bound to Tyrosine residues on thyroglobulin. This is organified iodine stays within the follicles and can not “wash out”. However, if the iodine is not organified, then it will wash out as demonstrated by a perchlorate discharge test. A more sensitive version of this test is the “iodide-perchlorate” discharge test. (101-108)
Hypothyroidism From High Iodide Intake
High dietary iodine intake is common in Japan, and most patients are able to compensate and have no adverse effects. However, an occasional patients will be unable to compensate and will develop a high TSH from the suppressive effects of iodine on thyroid function. These patients harbor subclinical Hashimoto’s thyroditis.
In 1986, Dr. Junichi Tajiri studied hypothyroidism (high TSH) induced by high iodine intake in 22 Japanese patients. For about half the patients, the hypothyroidism was reversible merely by stopping the high iodine diet (i.e. iodine restriction). In the other half in which the hypothyroidism was irreversible, 9 out of 10 patients had a positive perchlorate discharge test, indicating underlying organification defect. Thyroid biopsies showed the reversible patients had milder lymphocytic thyroiditis (autoimmune thyroiditis), while those with irreversible hypothyroidism had more severe thyroid destruction. This indicates an organification defect is playing a major role in patients with lymphocytic thyroiditis (i.e Hashimoto’s Thyroiditis). Patients unable to compensate while ingesting a high iodine diet usually have subclinical autoimmune thyroiditis with an organification defect. The authors write:
The patients with reversible hypothyroidism had focal lymphocytic thyroiditis changes in the thyroid biopsy specimen, whereas those with irreversible hypothyroidism had more severe destruction of the thyroid gland. These results indicate the existence of a reversible type of hypothyroidism sensitive to iodine restriction and characterized by relatively minor changes in lymphocytic thyroiditis histologically. Attention should be directed to this type of hypothyroidism, because thyroid function may revert to normal with iodine restriction alone. (102)
Methimazole Thyroid Blocking Drug
Methimazole (MMI), the first line thyroid blocking drug, works by irreversibly binding to and blocking function of the TPO enzyme, thus inhibiting organification of iodine. This inability to organify induced by Methimazole bears a similarity to Hashimotos’ Thyroiditis which also has organification defect. In addition, Methimazole inhibits the DUOX enzyme, thus inhibiting hydrogen peroxide formation, a beneficial feature which prevents excess hydrogen peroxide damage to the thyrocytes.
When starting MMI for Graves’ Disease, it takes about six weeks (plus or minus two weeks) for thyroid hormone levels to normalize. It takes this amount of time for all the preformed thyroid hormone stored in follicles to be secreted and metabolized. Adverse side effects include rash and agranulocytosis and are dose related. Agranulocytosis is serious and can induce fatal immunosuppression. MMI is about 10 times more potent than PTU (Propylthiouracil) in blocking thyroid function. Although MMI partially inhibits the DUOX enzyme which generates hydrogen peroxide, it does not block the D1 Deiodinase enzyme which converts T4 to T3 in the periphery, as does PTU, corticosteroids and Beta Blockers. MMI has a long duration of action allowing for once a day dosing.(16)(65)(99-100)
A major difference in thyroid blocking function of Methimazole vs. Potassium Iodide (KI) or Lithium Carbonate is this: Methimazole (MMI) does not block release of thyroid hormone from the thyroid gland. However, both KI and Lithium block release of thyroid hormone from the thyroid gland. This feature is useful in some patients resistant to MMI, as it provides a different mechanism of action which may remain effective in spite of resistance to MMI. In Painless Thyroiditis (PT), MMI is ineffective, and other agents PTU, Corticosteroids and Beta Blockers are more useful, as these inhibit D1 Deiodinease and block conversion of T4 to T3.
Loss of Auto-Regulation in Hashimoto’s
As mentioned above, the thyroid gland in Hashimoto’s patients has lost the autoregulation needed to escape from suppressive effects of KI dietary excess, rendering these patients more sensitive to “Iodine Blockade” described by Wolff and Chaikoff as the suppressive effect of iodide on thyroid function. In normal thyroid glands, down regulation of the NIS symporter RNA by iodide as well as a direct inhibition of NIS within the basal membrane leads to eventual escape from the suppressive effects of KI within two days. When autoregulatory activity is functioning normally, the thyroid compensates for the excess iodide by reducing iodide uptake and concentration at the NIS at the basolateral membrane.
Although the normal healthy thyroid gland may escape form the inhibitory effects of excess iodide, there is no similar escape from thyroid blockade with Methimazole or Lithium.(74-75)
As mentioned above, the second antibody in Hashimoto’s Disease is the anti-thyroglobulin antibody. Additional antibodies against the NIS (sodium iodine symporter), and against Pendrin have also been discovered. However, lab tests for NIS and Pendrin are not yet available for clinicians.
Graves’ Disease, TSI and TRAb Antibodies
In Graves’ Disease, the TSI and TRAb antibodies stimulate the TSH receptor causing the signs and symptoms or Graves’ hyperthyroidism. About 70 per cent of Graves’ Disease patients will also have TPO and thyroglobulin antibodies. These are the Hashimoto’s Antibiodies. Indeed, some authors believe Graves’ and Hashimotos’ are two extremes of the same disease process with different manifestations.
Methimazole-Antithyroid medication
Methimazole (MMI) works by irreversibly inactivating the thyroid peroxidase enzyme (TPO), thus preventing organification, a necessary step for thyroid hormone synthesis. In 1999, working in vitro, Dr Sugawara found MMI increases TPO mRNA, resulting in greater amounts of TPO enzyme in the thyroid gland. Thus, upon stopping or decreasing the MMI drug, there may be a rebound phenomenon with worsening hyperthyoidism caused by Painless Thyroiditis as TPO activity is increased, as discussed by Dr. Ken Okamura in 2022.(120)
In this event, it would be prudent to supplement with selenium and magnesium to prevent thyroid damage from excess hydrogen peroxide. This is discussed further in another chapter on Iodine Treatment of Graves Disease Part Two. (8-10)(52-53)
Genetic Mutation in Thyroperoxidase
Failure to organify iodine can be found in those harboring genetic mutations in the thyroperoxidase enzyme. In these cases, iodide in the thyroid gland cannot be oxidized and/or bound to the thyroglobulin protein.(8)
Selenium, Selenoproteins and Hydrogen Peroxide
The normal thyroid gland has a high selenium content due to high concentration of selenoproteins (glutathione peroxidases and deiodinases) which are anti-oxidants protecting the thyrocytes from oxidative damage associated with hydrogen peroxide production, needed for thyroid hormone biosynthesis.
Iodination of thyroglobulin, also called organification, is the key step of thyroid hormone biosynthesis. It is catalyzed by thyroid peroxidase (TPO) and occurs within the follicular space at the apical plasma membrane. Hydrogen peroxide generating enzymes, called DUOX, Dual Oxidase, are also found at this same location, at the villous apical membrane of the thyrocyte just within the follicular lumen. Both enzyme systems (TPO and DUOX) are needed for the organification of iodine to Tyrosine residues in the thyroglobulin. Various thyroid pathologies, including Hashimoto’s thyroiditis can be explained by overproduction and lack of degradation of hydrogen peroxide (H2O2) causing damage to the thyrocyte structures. (11-16)
Wolff Chaikoff Effect – Excess Iodine Intake Inhibits Organification
Seventy four years after it was first described, the “Wolff-Chaikoff” effect is still not well understood. The best way to describe the Wolff-Chaikoff effect is the inhibition by iodide of its own organification. This effect describes the inhibition of hydrogen peroxide (H2O2) generation caused by iodide itself. The intake of excess iodide limits the oxidation and binding of I2 (iodine) to thyroglobulin because of the reduced availability of hydrogen peroxide at the apical membrane. As mentioned previously, in Hashimoto’s thyroiditis, autoregulatory functions are lost, and these patients are more sensitive to the inhibitory effect of iodine excess on thyroid function. Give them iodine and the TSH will go up. The same is true for Graves’ Disease. Normal healthy people have no trouble escaping from the inhibitory effects of iodide. After a few days, TSH returns to normal. This is called the “Escape from the Wolff-Chaikoff Effect”. The mechanism for escape is generation of iodolactones, and reduction in NIS activity and NIS mRNA.
On occasion, an apparently normal healthy person will visit my office because they have a very high TSH after consuming an iodide/iodine supplement from the health food store. These people have subclinical (euthyroid) autoimmune thyroid disease and are unable to escape from the inhibitory effects of the iodide (the Wolff-Chaikoff Effect). In 2002, Dr K. Markou discusses this same exact point, writing:
However, in a few apparently normal individuals, in newborns and fetuses, in some patients with chronic systemic diseases, euthyroid patients with autoimmune thyroiditis, and Graves’ disease patients previously treated with radioimmunoassay (RAI), surgery or antithyroid drugs, the escape from the inhibitory effect of large doses of iodides is not achieved and clinical or subclinical hypothyroidism ensues.(18)
TSH Stimulates Hydrogen Peroxide Generation
In 1988, Dr Bernard Corvilain studied the Wolff-Chaikoff effect in dog thyroid slices in vitro, finding elevated TSH stimulates hydrogen peroxide generation and organification. Excess iodide had the opposite effect, greatly inhibiting hydrogen peroxide generation as well as organification via reduction of intracellular signaling from TSH (The Wolff-Chaikoff Effect), writing:
In dog thyroid slices thyrotropin [TSH] and carbamylcholine greatly enhance protein iodination and H2O2 generation. The action of thyrotropin [TSH] is … mediated by cyclic AMP. This suggests that the effect of carbamylcholine is mediated by the two intracellular signals generated by the Ca++ phosphatidylinositol cascade: Ca++ and diacylglycerol. The Wolff-Chaikoff effect is the inhibition by iodide of its own organification…In dog thyroid slices, iodide greatly inhibited H2O2 generation stimulated by thyrotropin [TSH] and by carbamylcholine. Iodide decreased the production of intracellular signals induced by TSH.(17)(116)
Note: carbamycholine is a common drug used in ophthalmology to dilate pupils. Emphasis Mine.
Iodine Depletion – Paradoxical Toxic Effects of Acute Iodine
In 2000, Dr. Bernard Corvilain again studied the effect of iodine administration on H202 production in animal thyroid slices, finding iodine depletion is a condition in which acute iodine administration stimulates H202 production, rather than inhibits it. When iodine is acutely given to an iodine deficient animal, there is stimulation of H202, presumably to promote efficient oxidation and organification of the iodine into thyroid hormone. This stimulation of H202 explains the toxic effect of acute iodine administration in iodine depleted animals. Dr. Bernard Corvilain writes:
In comparison with conditions in which an inhibitory effect of iodide on H2O2 generation is observed [Wolff-Chaikoff Effect], the stimulating effect was observed for lower concentrations and for a shorter incubation time with iodide. Such a dual control of H2O2 generation by iodide has the physiological interest of promoting an efficient oxidation of iodide when the substrate is provided to a deficient gland [iodine deficient gland] and of avoiding excessive oxidation of iodide and thus synthesis of thyroid hormones when it is in excess. The activation of H2O2 generation may also explain the well described toxic effect of acute administration of iodide on iodine-depleted thyroids. (17B) Emphasis Mine
Thyroid Hormone Synthesis Courtesy of Szanto 2019 Fig 3. Szanto, Ildiko, Marc Pusztaszeri, and Maria Mavromati. “H2O2 metabolism in normal thyroid cells and in thyroid tumorigenesis: focus on NADPH oxidases.” Antioxidants 8.5 (2019): 126.
Hydrogen Peroxide Metabolism
in 2019, Dr Ildiko Szanto reviewed the role of hydrogen peroxide in normal thyroid metabolism in organification of iodine and production of thyroid hormone. Excessive hydrogen peroxide generation is not only mutagenic, it is also at the core of most other thyroid pathologies, such as goiter, nodules and auto-immune thyroid disease, writing:
The synthesis of thyroid hormones… utilize hydrogen peroxide (H2O2) as an oxidative agent. Hydrogen peroxide is contained within the lumen of the thyroid follicles which are considered as the functional units of the thyroid gland where hormone synthesis, storage and release take place. The follicles are formed by a monolayer of polarized epithelial cells, termed thyrocytes, that surround the central lumen of the follicle. The lumen is delimited by the apical surface of thyrocytes that are connected to each other by tight junctions preventing “leaking” of the lumen content into the extra-follicular space. The outer side of the follicle is sealed by the basolateral plasma membranes of thyrocytes. The lumen is filled by a protein-rich substance termed “colloid”, mainly consisting of thyroglobulin (TG), a 660kDa thyroid-specific protein. TG accounts for approximatively 50% of the protein content of the thyroid gland and serves as the precursor of thyroid hormones. Thyroid hormone synthesis requires the oxidative iodination of specific tyrosine residues of TG, a process termed “iodide organification”. Oxidative iodination is catalyzed by the enzyme thyroid peroxidase (TPO). This process is a key step in thyroid hormonogenesis requiring an appropriate amount of H2O2 for oxidation…The major source of H2O2 in the thyroid follicle is the isoform DUOX2. In contrast to DUOX2-generated physiological reactive oxygen species (ROS) production, pathologically elevated H2O2 levels are linked to thyroid carcinogenesis resulting from enhanced mitogenic receptor signals or oncogene activation. (22)
Regulation of H202 generation by TSH and Iodine
The hydrogen peroxide generating system is called DUOX2, and as one might expect DUOX2 is up-regulated by TSH in two ways:
1) TSH increases messenger RNA to increase production of the DUOX2 protein.
2) TSH increases the DUOX2 enzymatic activity.
Iodo-Lactones Mediate Autoregulation
Quite the opposite of TSH, excess iodide inhibits DUOX2 mediated production of hydrogen peroxide. This is the Wolff-Chaikoff Effect, first described in 1948, i.e. Iodine inhibits its own organization. This is a form of thyroid autoregulation which is mediated by Iodolactones and iodoaldehydes formed when molecular iodine reacts with lipids. Iodolactones not only mediate thyroid autoregulation, they also play an important role as anti-cancer agents in prevention and treatment of breast cancer. Breast cancer cells have the NIS, sodium iodide transporter, and actively take up iodine which reacts with lipids to form iodolactones, the anticancer agent. See the chapter on prevention and treatment of breast cancer with iodine. (77-83)
Dr Ildiko Szanto writes:
DUOX2-mediated H2O2 production is inhibited by excess iodine leading to decreased TPO activity and reduced incorporation of iodine into TG [thyroglobulin]. This inhibitory effect of iodine on its own organification was already described in 1948 and was named the “Wolff-Chaikoff effect” after the authors of the original paper.(22)(62-63)
Graves TSH Receptor Antibodies DO NOT increase H202 production
In Graves’ Disease, elevated antibodies to the TSH receptor (TSI and TRAb) stimulate the thyroid gland to “go into overdrive”, producing massive quantities of thyroid hormones. The elevated T3 and T4 suppress the TSH which will be quite low (suppressed) in Graves’ disease. One might assume this same TSH receptor stimulation increases H202 generation. This would be an incorrect assumption. While TSH hormone stimulates H202 generation, TSH receptor antibodies in Graves’ disease do not stimulate H202 production.(64)
This may explain why the difference in appearance of the thyroid gland in classical Graves’ Disease versus Hashimotos’. In Classical Graves’ Disease the thyroid gland is initially smoothly enlarged without nodularity or fibrosis, and typically, there are none of the inflammatory infiltrative changes typically found in Hashimotos’ thyroiditis. In Graves’ disease, the TSH is suppressed. This essentially turns off excess hydrogen peroxide generation. Of course, over time, after many years of anti-thyroid drug treatment and repeated bouts of thyroiditis, the morphology of the thyroid gland changes. Microscopic study may show appearance of inflammatory infiltrates, especially if there are elevated TPO and Thyroglobulin antibodies as seen in 70 per cent of Graves’ Disease patients.
Undulating Course of Graves’ with Relapse and Remission
How can we explain the undulating course of Graves’ Disease with cycles of relapse and remission. During remission obtained with anti-thyroid drugs such as Methimazole or Potassium Iodide, the TSH may soar to high levels, stimulating excess H202 production. At this stage, if anti-thyroid drug dosage is reduced, this may trigger thyroiditis. The inflammatory effect of excess H202 causes rupture of follicles with release of preformed thyroid hormone and “relapse of hyperthyroidism”. This is called Painless Thyroiditis (PT) The relapse of hyperthyroidism with high Free T3 and T4 which will in turn, suppress the TSH which “turns off” H202 production. This allows healing and the cycle repeats.
Block and Replace
TSH elevation along with its stimulation of H202 generation may occur during over-treatment with thyroid blocking drugs such as Methimazole (MMI), KI or Lithium Carbonate which inhibit thyroid function and drive up TSH. This TSH elevation stimulates hydrogen peroxide production which may damage the thyroid gland. “Block and Replace” would then be useful to maintain a suppressed TSH which prevents thyroid stimulation and damage from excess H202. Maintaining a higher Anti-Thyroid Drug (ATD) dosage is beneficial, as reducing dosage at this stage may provoke thyroiditis. Remember, Methimazole not only irreversibly blocks the TPO enzyme, it also inhibits hydrogen peroxide generation.
Figure 1 Thyroid hormone biosynthesis, secretion and major signaling pathways in thyrocytes. AC, adenyl cyclase; cAMP, cyclic adenosine monophosphate; DAG, diacylglycerol; DEHAL1, dehalogenase 1; DIT, diiodotyrosine; DUOX, dual oxidase; DUOXA, dual oxidase maturation factor; IP3, inositol trisphosphate; MIT, monoiodotyrosine; NIS, sodium-iodide symporter; PDS, pendrin; PKA, protein kinase A; PKC, protein kinase C; PLC, phospholipase C; T3, triiodothyronine; T4, thyroxine; TG, thyroglobulin; TPO, thyroid peroxidase; TSH, thyrotropin; TSHR, thyrotropin receptor . Courtesy of (Ohye, 2010)
Dr. Hidemi Ohye Explains Production of Thyroid Hormone
In 2010, Dr. Hidemi Ohye discusses the production of thyroid hormone, again explaining “thyroid-stimulating antibody found in patients with Graves’ disease does not appear to stimulate H2O2 generation.” In addition, both Methimazole and PTU inhibit H202 generation. Dr. Hidemi Ohye writes:
Iodide is actively transported into thyrocytes by a sodium/iodide symporter (NIS) on the basolateral membrane and to the follicular lumen by, in part, pendrin…at the apical membrane. Iodide is rapidly oxidized by TPO in the presence of H2O2 resulting in covalent binding to the tyrosyl residues of thyroglobulin (Tg) on the luminal side of the apical membrane. This step produces monoiodotyrosine (MIT) and diiodotyrosine (DIT). Then only properly spaced MIT and DIT in Tg participate in the coupling reactions to form thyroxine (T4) and triiodothyronine (T3); this reaction is also catalyzed by TPO with H2O2. The source of thyroid H2O2 is DUOX2 expressed in the apical plasma membrane coordinated with DUOXA2. Thyroid hormones are released into the circulation after digestion of Tg… Thyroid hormone formation is predominantly regulated by thyrotropin (TSH). The binding of TSH to the TSH receptor activates both Gs and Gq proteins. The former activates the growth regulation, differentiation and thyroid hormone secretion, whereas the latter activates H2O2 generation and iodide binding to protein through the phospholipase C-dependent inositol phosphate Ca2þ/diacylglycerol pathway…. Methimazole and propylthiouracil [PTU] inhibit NADPH oxidase activity [inhibits H202 generation]… Thyroid-stimulating antibody found in patients with Graves’ disease does not appear to stimulate H2O2 generation. (16)(64) Emphasis Mine.
TSH Controls H202 Levels
Note in the above quote, Dr. Hidemi Ohye explains how TSH controls thyroid gland growth, regulation, differentiation and thyroid hormone secretion, as well as H2O2 generation, and iodide binding to protein (organification). Higher TSH stimulates all these steps in thyroid hormone synthesis as well as thyroglobulin production, sodium Iodide Symporter activity and Thyroid Peroxidase protein expression.
Iodide Excess Reduces H202 Generation
Dr. Hidemi Ohye goes on to explain how Iodide controls H202 levels. While TSH stimulates greater H202 production, Iodide excess reduces H202 production. This is done through post-transcription change in the DUOX molecule. Note: DUOX is the Dual Oxidase system which generates hydrogen peroxide. Dr. Hidemi Ohye writes:
Iodide controls H2O2 generation in thyroid cells. Morand et al. have studied the effect of KI on H2O2 generation in porcine thyroid follicles, the most physiological thyroid culture system. They exposed follicles to 1 mmol/L KI for two days under cAMP stimulation and showed reduction in H2O2 production without affecting DUOX mRNA levels. Post-transcriptional change of the DUOX molecule by KI appears to be responsible for the decreased H2O2 generation. (16)
Wolff–Chaikoff Effect Protects Against Iodine Excess
In the normal thyroid gland, the Wolff–Chaikoff effect protects thyrocytes from iodide excess by inhibiting iodide organification, leading to discharge of iodide which cannot be organified. After two days, the autoregularoty features of the thyroid gland down regulate iodine uptake by the NIS, preventing furthur uptake of iodine. So, two things are happening. One, iodide excess inhibits organification by suppressing H202 generation by DUOX. Two, excess iodide has an inhibitroy effect on the NIS which then suppresses uptake and concentrarion of iodide, also called the “Escape from the Wolff Chaikoff Effect”. This is the same mechanism used when passing out iodine capsules after a nuclear accident. When a 65 mg potassium iodide capsule is distributed to the population surrounding a nuclear accident, the “Escape from the Wolff–Chaikoff Effect” prevents uptake of radioactive iodine into the thyroid gland, protecting the population from increased risk of thyroid cancer.
The Defective Wolff-Chaikoff Effect
Dr. Hidemi Ohye speculates that some animals and humans have a defective Wolff Chaikoff Effect. Instead of inhibiting hydrogen peroxide generation, excess iodine intake INCREASES IT ! An example of this is the iodine depleted animal or human. Dr. Hidemi Ohye also speculates these susceptible animals or humans, thyroid damage from H202 production due to defective antioxidant system may play a role in etiology of autoimmune thyroid disease. Damaged TPO and thyroglobulin proteins may then serve as antigen for auto-immune attack. Selenium or Magnesium deficiency may render the antioxidant system defective. Dr. Hidemi Ohye writes:
The animal experiments [in iodine depleted animals] suggest that susceptible hosts have defective Wolff–Chaikoff effect allowing them to generate H2O2 in response to increased iodide influx, which ordinarily should not happen as seen in the non-susceptible mouse. Thus, abnormality of thyroid H2O2 generation in response to high iodide may play a role in the development of Hashimoto’s thyroiditis in susceptible individuals. Whether iodide-mediated H2O2 generation is driven by activated DUOX or NOX4 [ H202 generating enzymes] or defective antioxidants [selenium deficiency] has yet to be studied.(16)(64-65)
Note: studies in iodine depleted animals shows exactly what is described above. Rather than inhibiting H202 production, refeeding iodine to an iodine depleted animal causes an increase in H202 production, which damages the TPO and Thyroglobulin leading to antigenicity. This supports the hypothesis iodine deficiency is the etiology of autoimmune thyroid disease. Here, I would add the thyroid gland in Graves’ Disease has very high radio-iodine uptake, with a much higher iodine requirement to keep up with massive thyroid hormone production, and therefore may mimick an iodine depleted thyroid which also has very high radio-iodine uptake. (115-117)
Graves’ Antibodies DO NOT Stimulate Hydrogen Peroxide !
As mentioned above, TSH stimulates all steps in thyroid hormone synthesis including generation of hydrogen peroxide (H202). In 1991, Dr Eric Laurent studied human thyroid slices in vitro, and surprisingly, found Graves’ Disease antibodies do not stimulate hydrogen peroxide generation. This makes sense to me. If Graves antibodies did stimulate excess hydrogen peroxide, this would lead to severe thyroid damage, chronic thyroiditis and early complete destruction of the thyroid gland similar to myxoedematous cretinism described in Zaire, Africa. This would also lead to immune cell infiltration and rapid complete destruction of the thyroid gland. Instead, in early untreated Graves’ disease, the gland is smoothly enlarged without evidence of chronic changes. Dr Eric Laurent writes:
The effects of thyroid-stimulating antibodies (TSAb) and of thyrotropin (TSH) were compared…The patterns of the response curves of TSAb and TSH on cyclic AMP accumulation were different, suggesting that different mechanisms may be involved. In addition, unlike TSH, TSAb were not able to stimulate H2O2 generation, which in human tissue mainly depends on the activation of the phosphatidylinositol-Ca2+ cascade. … In conclusion, our results show that TSAb do not share all the metabolic actions of TSH on human thyroid tissue. (64)
As mentioned above, under normal circumstances, Hydrogen Peroxide generation is inhibited by excess iodine, and partially inhibited by Methimazole and PTU (propylthiouracil). Hydrogen Peroxide generation is stimulated by TSH, but not Graves’ TSH Receptor antibodies. (65-69)
Dr. Guy Abraham: How to Reverse Auto-Immune Thyroid Disease
Dr. Guy Abraham, inventor of the Iodoral tablet, explains iodine deficiency causes increased TSH. This stimulates excess hydrogen peroxide which damages TPO and thyroglobulin causing Hashimoto’s thyroiditis. The mechanism of excess H2O2 production is caused by combined iodine and magnesium deficiencies resulting in low levels of iodinated lipids and high cytosolic calcium, respectively. The excess hydrogen peroxide damages structures in closest proximity, TPO and the substrate thyroglobulin (Tg). These damaged proteins stimulate autoimmunity, thus explaining the origin of Hashimotos’ Disease from iodine deficiency. In Dr. Abraham’s opinion, supplementing with iodine (12.5 mg/day or above) and magnesium (1200 mg per day) should reverse auto-immune thyroiditis, both Hashimotos’ and Graves’ disease. Dr. Guy Abraham writes:
We would like to propose a mechanism for the oxidative damage caused by low levels of iodide combined with antithyroid drugs: inadequate iodide supply to the thyroid gland, aggravated by goitrogens [a substance that blocks iodine uptake, worsening the iodine deficiency], activates the thyroid peroxydase (TPO) system through elevated TSH, low levels of iodinated lipids, and high cytosolic free calcium, resulting in excess production of H2O2. …This H2O2 production is above normal due to a deficient feedback system caused by high cytosolic calcium resulting from magnesium deficiency and low levels of iodinated lipids which requires for their synthesis iodide levels two orders of magnitude greater than the RDA for iodine. Once the low iodide supply is depleted, TPO in the presence of H2O2 and organic substrate reverts to its peroxydase function which is the primary function of haloperoxydases, causing oxidative damage to molecules nearest to the site of action: TPO and the substrate thyroglobulin (Tg). Oxidized TPO and Tg elicit an autoimmune reaction with production of antibodies against these altered proteins with subsequent damage to the apical membrane of the thyroid cells, resulting in the lymphocytic infiltration and in the clinical manifestations of Hashimoto’s thyroiditis…In laboratory animals prone to autoimmune thyroiditis, the genetic defect may be in the production of H2O2 in excess of what is needed. The iodination of tyrosine residues by TPO requires the presence of Tg, H2O2, and iodide. The supply of H2O2 comes from the NADPH oxydase system. This system is inhibited by certain iodinated lipids and is enhanced by cytosolic free calcium Ca++. The equation for organification of iodide by TPO is displayed in Figure 1, together with the feedback system controlling the production of H2O2. The logical deduction from this equation is that increased cytosolic free calcium will cause an excess of H2O2. Increased levels of iodinated lipids, on the other hand, would limit the production of H2O2. How much iodide is required for the production of iodinated lipids? In 1976, Rabinovitch, et al reported their results regarding the effect of three levels of iodide supplementation on the production of iodinated lipids in the thyroid glands of dogs: low iodide diet, normal iodide diet, and high iodine diet. The dogs were kept on those diets for six weeks. Iodinated lipids in the plasma membrane and in the cell total lipids were observed only in the dogs receiving the high iodide diet. What about human subjects? In 1994, Dugrillon, et al reported for the first time the presence of delta lactone [delta iodolactone] in a human thyroid, following the ingestion of 15 mg iodide/day for 10 days in the host. It was the first time this biologically active iodolipid was isolated from human thyroid glands. The amount of iodide the host received was 100 times the RDA, but it is the amount of iodine/iodide we recommended for orthoiodosupplementation… Dugrillon, et al stated, “These results demonstrate for the first time that delta-iodo-lactone is present in iodide-treated human thyroid.” Magnesium deficiency, which is prevalent in the US population, results in increased levels of cytosolic free calcium. Intracellular free calcium levels above the normal range are cytotoxic causing calcification of mitochondria and cell death. The cell membrane possesses an ATP-dependant calcium pump that keeps intracellular levels of free ionized calcium within narrow limits. This calcium pump is magnesium-dependent for normal function. Magnesium deficiency results in a defective calcium pump and intracellular accumulation of ionized calcium. Inadequate iodine/iodide intake below orthoiodosupplementation results in decreased levels of delta-iodo-lactone. Combined magnesium and iodine/iodide deficiency based on the concept of orthoiodosupplementation are the basic factors involved in the
oxidative damage caused by excess H2O2 and reactive oxygen species. If this proposed mechanism is valid, orthoiodosupplementation, combined with magnesium intake between 800-1,200 mg/day, a daily amount this author recommended 21 years ago for magnesium sufficiency, should reverse autoimmune thyroiditis. This nutritional approach is also effective in Graves’ autoimmune thyroiditis as previously discussed.(70-72)
To my knowledge, as yet, there are no clinical trials examining the above hypothesis using iodine and magnesium to reverse auto-immune thyroid disease as proposed by Dr Abraham.
The Role of Iodinated Lipids – Iodolactones
Iodo-lipids (iodolactones) are made by reaction of excess iodine with lipids, and are thought to be involved in thyroid auto-regulation by inhibiting H202 production in the thyroid gland. Iodolipids are also thought active in extra-thyroidal tissues with NIS activity. For example, iodolactones generated by iodine intake may prevent or treat breast cancer. (79-85)
This is a quote from Drs. Dunn and Dunn in 2001 who reviewed iodide metabolism in the thyroid, explaining iodinated lipids increase linearly with addition of iodide, and inhibit hydrogen peroxide generation:
Iodinated lipids also occur in the thyroid, especially after high doses of iodide. One of these, 2-iodohexadecanal increases linearly with iodine addition. It inhibits NADPH oxidase, and lipid iodination appears to decrease H2O2 production, and may thus retard Tg iodination as well. (86-87)
In 2014, Drs. Mario Nava-Villalba and Carmen Aceves studied 6-iodolactone, finding that it is not only involved in thyroid auto-regulation, it is also involved in extra-thyroidal tissues containing NIS (sodium iodide symporter) such as mammary gland, prostate, colon, or the nervous system, writing:
An iodinated derivative of arachidonic acid, 5-hydroxy-6-iodo-8,11,14-eicosatrienoic acid, δ-lactone (6-IL) has been implicated as a possible intermediate in the autoregulation of the thyroid gland by iodine. In addition to antiproliferative and apoptotic effects observed in thyrocytes, this iodolipid could also exert similar actions in cells derived from extrathyroidal tissues like mammary gland, prostate, colon, or the nervous system. In mammary cancer (solid tumors or tumor cell lines), 6-IL has been detected after molecular iodine (I2) supplement, and is a potent activator of peroxisome proliferator-activated receptor type gamma (PPARγ). These observations led us to propose I2 [molecular iodine] supplement as a novel coadjutant therapy which, by inducing differentiation mechanisms, decreases tumor progression and prevents chemoresistance. Some kinds of tumoral cells, in contrast to normal cells, contain high concentrations of arachidonic acid, making the I2 supplement a potential “magic bullet” that enables local, specific production of 6-IL, which then exerts antineoplastic actions with minimal deleterious effects on normal tissues. (84-97)
H202 and Various Thyroid Pathologies
In 2008, Dr Song explains various thyroid pathologies are caused by either overproduction of hydrogen peroxide, or its lack of degradation due to selenium deprivation and consequent GSH [glutathione] peroxidase depletion, thus explaning the etiology of auto-immune thyroid disease, goiter, thyroid nodules, thyroid cancer etc.
Excess H202 is Carcinogen and Killer
Dr. Song reminds us excess H2O2 in thyroid cells is a carcinogen and “a killer”. Dr. Song then discusses the etiology of myxedematous endemic cretinism in Zaire Africa, a form of thyroid destruction after birth asociated with iodine deficiency which stimulates elevated TSH and consequently H2O2 generation, combined with selenium deficiency (causing decreased GSH peroxidase and thioredoxin reductase activity), combined with dietary thiocyanate. Dr. Song proposes a similar mechanism for pathophysiology of thyroiditis, and proposes selenium dietary supplementation for prevention and treatment of thyroiditis. Dr. Song writes:
In thyrocytes of most species, including humans, TSH and its receptor activates…H2O2 generation, iodide binding to proteins [organification], and thyroid hormone formation, and secretion…In all species studied, iodide at high concentrations presumably through an iodinated lipid, iodohexadecanal [6-iodo-lactone], inhibits H2O2 generation (the Wolff-Chaikoff effect) and adenylate cyclase…H2O2 in various cell types, and presumably in thyroid cells, is a signal, a mitogen, a mutagen, a carcinogen, and a killer…It is proposed that various pathologies can be explained, at least in part, by overproduction and lack of degradation of H2O2 (tumorigenesis, myxedematous cretinism, and thyroiditis) and by failure of the H2O2 generation or its positive control system (congenital hypothyroidism)……We have repeatedly suggested that the important generation of H2O2 in thyroid cells might account for mutagenesis and the important generation of nodules in the thyroid. This would also explain in part why more nodules are found in iodine-deficient areas…Myxedematous endemic cretinism [Zaire, Africa], caused by thyroid destruction after birth, has been linked to low iodine supply in early life, leading to intense stimulation [by elevated TSH] and presumably H2O2 generation, to passage from low O2 to high O2 at birth, to selenium deficiency, and thus to decreases in GSH peroxidase and thioredoxin reductase activity and to dietary thiocyanate…Interestingly, a similar scenario has been proposed for the physiopathology of thyroiditis. Selenium dietary supplementation has therefore been proposed for prevention and treatment of thyroiditis and has indeed alleviated it. (4) Emphasis Mine.
Above image courtesy of Dr Robyn Murphy, (2016) (76)
Dr. Robyn Murphy Clears Everything Up
In 2016, Dr. Robyn Murphy agrees with Dr. Guy Abraham and Dr. Song suggesting iodine deficiency as a causative factor in patients with auto immune thyroid disease. Firstly, Dr. Murphy points out the observed increase in auto-immune thyroiditis after introduction of salt iodination programs. This can be understood as refeeding iodine to an iodine deficient population. Chronic iodine deficiency creates a loss of autoregulation in the thyroid leading to excess hydrogen peroxide generation when iodine is re-introduced. This was confirmed by Dr. Corvilan’s and Dr. Cohen in vitro studies using thyroid slices and in mice. (116-117)
Dr. Robyn Murphy goes on to describe the train of events in the production of thyroid hormones, explaining how iodide deficiency is causative in etiology of auto-immune thyroid disease. Iodide intake of at least 1.5 mg per day (1,500 micrograms) is required to produce enough iodo-lactones to inhibit hydrogen peroxide generation. Dr. Murphy writes:
In iodine deficiency, the loss of negative feedback on H2O2 production has been implicated in thyroid dysfunction and as a possible mechanism in the generation of autoantibodies [Hashimoto’s Antibodies]…The sodium-iodide symporter (NIS) transports iodide (I−) into the thyrocyte to be organified to iodine (I2) and bound to thyroglobulin (TG) by thyroid peroxidase (TPO) and hydrogen peroxide (H2O2). The iodine–thyroglobulin complexes then combine to form thyroid hormones. The nicotinamide adenine dinucleotide phosphate (NADPH) oxidase system is upregulated by intracellular calcium to generate H2O2, a reactive oxygen species (ROS). To prevent excess H2O2, iodolactones negatively inhibit NADPH oxidase, and glutathione peroxidase degrades H2O2…Iodinated lipids provide negative feedback on NADPH oxidase to reduce H2O2 and oxidative damage to the thyrocyte…Studies suggest that when iodine intake is above 1.5 mg daily, TPO synthesizes iodolactones. TPO facilitates the iodination of arachidonic acid and other polyunsaturated fatty acids to produce iodolactones. Research shows that measurable concentrations of δ-iodolactone are found in the thyroid tissue, prevent excess iodide uptake and the generation of H2O2 by NADPH oxidase, and regulate thyroid function. However, in iodine deficiency, iodolactones are obsolete, and the thyroid gland is susceptible to oxidative damage and loss of cell-cycle control…In chronic iodine deficiency, compensatory mechanisms fail to maintain iodine concentration, and the thyroid is susceptible to damage. As iodine concentrations decrease, the pituitary gland secretes thyroid-stimulating hormone (TSH) to induce NIS expression. TSH remains high, and the NIS fails to shut down, predisposing the gland to excess iodine during supplementation, a phenomenon found in individuals with preexisting autoimmune thyroiditis (AIT). In addition, thyroid activity remains high, leading to hyperplasia and goiter. Researchers in cross-sectional studies reported that patients with nodular goiters often have an increase in thyroid antibodies, which is more common in iodine deficiency. These observations suggest that immune stimulation to thyroid proteins occurs during iodine deficiency and may be a causative factor in the development of thyroid autoimmunity...In iodine deficiency, the thyroid is susceptible to oxidative damage. As TSH stimulates NIS and TPO activity, low levels of iodinated lipids with high cytosolic free calcium allow for excess H2O2 to be produced [Dr Abraham’s same point]. Iodine alone fails to initiate immune activation; rather, experimental studies show that iodine in the presence of inflammatory cytokines [associated with increased hydrogen peroxide] augments immune function… In the presence of excess H2O2, oxidative damage to TPO [Thyroperoxidase] and TG [Thyro globulin] leads to antigen presentation and lymphocytic infiltration, which facilitates the production of anti-TPO and anti-TG antibodies. This mechanism may account for the prevalence of AIT [auto imune thyroiditis] upon iodine supplementation through USI [United States Iodination] programs…The lowest concentrations of iodine are found in individuals with thyroid autoimmunity. In 13 patients with overt symptoms of hypothyroidism and AIT, an average of 2.3 mg of iodine was found in the thyroid, as compared with 10 mg in healthy subjects. On the basis of data derived from the National Health and Nutrition Examination Survey, although iodine intake in the United States continues to decrease, the prevalence of AIT continues to increase. This correlation suggests that iodine deficiency may be a causative factor in patients with AIT [auto-immune thyroiditis] . (76)
TSH Stimulates Conversion of T4 to T3
In 1990, Dr. Kohrle found TSH receptor stimulation controls the activity of intra-thyroidal D1 Deiodinase which converts T4 to T3. Thus, high TSH receptor stimulation as found in severe forms of Graves’ Disease increases the peripheral FreeT3/ FreeT4 ratio. High Free T3 blood level is an indicator of a more severe form of Graves’ thyrotoxicosis. Quite the opposite in inflammatory thyroiditis, the Free T4 predominates rather than the Free T3, representing a useful method to differentitate the two entities.
Deiodinases D1, D2 and D3
D1 is present in all tissues of the body, and converts T4 to T3. D2 is present in the pituitary only, and converts T4 to T3 in the pituitary. D3 is present in all tissues and converts T4 to inactive reverse T3.
D1 Deiodinase, which converts T4 to T3 in the periphery, is inhibited by amiodarone, thereby causing “tissue hypothyroidism”, and blocks direct effect of T3 on heart muscle cells, an effect beneficial as an anti-arrythmia agent. Peripheral conversion of T4 to T3 is also inhibited by PTU, beta blocker drugs (propranolol, atenolol), and cortocosteroids, but NOT Methimazole (MMI). In 2022, Dr G Bereda writes: “PTU (but not MMI) also inhibits the peripheral conversion of T4 to T3.” (109-114)
In 1983, Dr Van Doom studied athyreotic mice finding initiation of MMI actually increased peripheral conversion of T4 to T3. This could explain initial paradoxical worsening of thyrotoxicosis symptoms when starting MMI which may last a day or so. This may be avoided by first starting the Beta Blocker which inhibits D1 conversion of T4 to T3. (110-111)
Dr. Kohrle writes:
TSH, the major signalling factor for the thyroid follicles, controls thyrocyte function…TSH is involved in the regulation of thyroidal uptake of small molecules and nutrients, intracellular transport of thyrocyte specific proteins, and in most of the steps of thyroid hormone synthesis, storage and release… Thyrocytes also express a highly active Type I iodothyronine 5′ deiodinase [D1 Deiodinase] which is controlled by TSH stimulated cAMP production. The thyrocyte specific 5′ deiodinase isozyme has marked influence on the amount of T3 secreted by the thyroid. This 5′ deiodinase isozyme shows most of the characteristics of the type I 5′ deiodinase found in liver and kidney and is also blocked by PTU, other 5′ deiodinase inhibitors [corticosteroids], and iodinated X-ray contrast agents such as iopanoic acid, which are occasionally used in thyrotoxicosis to inhibit thyroidal T3-production by this enzyme. (88)
Dr Margaret Rayman Explains Importance of Iron
TPO is a Heme Protein with Central Iron
As mentioned above, TPO, thyroperoxidase is directly responsible for organification of iodine into thyroglobulin. TPO is a heme enzyme containing a porphyrin ring with a central iron atom. Thus, thyroid hormone production is dependent on adequate iron stores. Iron supplementation to a ferritin level of 100 μg/l is suggested by Dr Margaret Rayman who writes in 2019:
It is important to recognise that low iron stores may contribute to symptom persistence in patients treated for hypothyroidism…An example is afforded by a small study in twenty-five Finnish women with persistent symptoms of hypothyroidism, despite appropriate L-T4 [Levothyroxine] therapy, who became symptom-free when treated with oral iron supplements for 6–12 months… all had serum ferritin <60 μg/l. Restoration of serum ferritin above 100 μg/l ameliorated the symptoms in two-thirds of the women. At least 30–50 % of hypothyroid patients with persisting symptoms despite adequate L-T4 therapy may, in fact, have covert ID [iron deficiency]…Patients with AITD or hypothyroidism should be routinely screened for ID [iron deficiency]. If either ID or serum ferritin below 70 μg/l is found, coeliac disease or autoimmune gastritis may be the cause and should be treated. (118-119)
Conclusion: The Nuclear Reactor Analogy
The thyroid gland can be compared to a nuclear reactor. Under normal working conditions the nuclear reactor generates energy safely. However when the cooling system fails, the reactor may go into catastrophic melt down. a “nuclear accident” with leakage of radioactivity. Similarly when the cooling system of the thyroid, the selenium based glutathione system is dysfunctional or when H202 generation is excessive, the thyroid gland undergoes “nuclear meltdown”, causing various thyroid pathologies, auto-immmune thyroid disease, thyroiditis, nodules, mutagenesis, and thyroid cancer.
Natural Thyroid Toolkit
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References
1) Shahid, Muhammad A., Muhammad A. Ashraf, and Sandeep Sharma. “Physiology, thyroid hormone.” StatPearls [Internet]. StatPearls Publishing, 2022.
The five steps of thyroid synthesis
Synthesis of Thyroglobulin: Thyrocytes in the thyroid follicles produce a protein called thyroglobulin (TG). TG does not contain any iodine, and it is a precursor protein stored in the lumen of follicles. It is produced in the rough endoplasmic reticulum. Golgi apparatus pack it into the vesicles, and then it enters the follicular lumen through exocytosis.
Iodide uptake: Protein kinase A phosphorylation causes increased activity of basolateral Na+-I- symporters, driven by Na+-K+-ATPase, to bring iodide from the circulation into the thyrocytes. Iodide then diffuses from the basolateral side to the apex of the cell, where it is transported into the colloid through the pendrin transporter.
Iodination of thyroglobulin: Protein kinase A also phosphorylates and activates the enzyme thyroid peroxidase (TPO). TPO has three functions: oxidation, organification, and coupling reaction.
Oxidation: TPO uses hydrogen peroxide to oxidize iodide (I-) to iodine (I2). NADPH-oxidase, an apical enzyme, generates hydrogen peroxide for TPO.
Organification: TPO links tyrosine residues of thyroglobulin protein with I2. It generates monoiodotyrosine (MIT) and diiodotyrosine (DIT). MIT has a single tyrosine residue with iodine, and DIT has two tyrosine residues with iodine.
Coupling reaction: TPO combines iodinated tyrosine residues to make triiodothyronine (T3) and tetraiodothyronine (T4). MIT and DIT join to form T3, and two DIT molecules form T4.
Storage: thyroid hormones are bound to thyroglobulin for stored in the follicular lumen.
Release: thyroid hormones are released into the fenestrated capillary network by thyrocytes in the following steps:
Thyrocytes uptake iodinated thyroglobulin via endocytosis
Lysosome fuse with the endosome containing iodinated thyroglobulin
Proteolytic enzymes in the endolysosome cleave thyroglobulin into MIT, DIT, T3, and T4.
T3 (20%) and T4 (80%) are released into the fenestrated capillaries via MCT8 transporter.[5]
Deiodinase enzymes remove iodine molecules from DIT and MIT. Iodine can be salvaged and redistributed to an intracellular iodide pool.[1][6][4]
2) Rousset, Bernard, et al. “Thyroid hormone synthesis and secretion, Chapter 2.” Endotext [Internet] (2015).
In this chapter “iodine” refers to the element in general, and
“molecular iodine” refers to I2.
“Iodide” refers specifically to the ion I-.
The normal thyroid maintains a concentration of free iodide 20 to 50 times higher than that of plasma, This concentration gradient may be more than 100:1 in the hyperactive thyroid of patients with Graves’ disease. The thyroid can also concentrate other ions, including bromide, astatide, pertechnetate, rhenate, and chlorate, but not fluoride (25;26).
Functional studies clearly show that NIS is responsible for most of the events previously described for iodide concentration by the thyroid. TSH stimulates NIS expression (39;40) and iodide transport (31;32). TSH exerts its regulatory action at the level of transcription through a thyroid-specific far-upstream enhancer denominated NUE (NIS Upstream Enhancer) that contains binding sites for the transcription factor Pax8 and a cAMP response element-like sequence. This original demonstration made on the rat NIS gene (41) has now been extended to human (42) and mouse (43) NIS genes. It has been suggested that TSH could also regulate NIS expression at post-transcriptional level (44). Data from TSH receptor-null mice (44-46) clearly show that TSH is required for expression of NIS.
Moderate doses of iodide in the TSH-stimulated dog thyroid inhibit expression of the mRNAs for NIS and TPO, while not affecting that for Tg and TSH receptor (40) . The decrease in thyroid iodide transport resulting from excess iodide administration (escape from the Wolff-Chaikoff effect, see further) is related to a decrease in NIS expression (40;47). Both NIS mRNA and NIS protein are suppressed by TGFb, which also inhibits iodide uptake (48;49). Reviews focus on NIS and its functional importance (50;51)
Iodide that enters the thyroid remains in the free state only briefly before it is further metabolized and bound to tyrosyl residues in Tg. A significant proportion of intrathyroidal iodide is free for about 10-20 minutes after administration of a radioactive tracer (81), but in the steady state, iodide contributes less than 1% of the thyroid total iodine. A major fraction of the intrathyroidal free iodide pool comes from deiodination of MIT and DIT; this iodide is either recycled within the thyroid or leaked into the circulation. Some data suggest that iodide entering the gland by active transport segregates from that generated by deiodination of Tg within the gland (82;83). Once in the thyroid, iodide is organically bound at a rate of 50 to 100% of the pool each minute (24;84). The proportion of an iodide load that is bound varies little, despite wide shifts in daily intake. In contrast, NIS activity is sensitive to both iodine availability and TSH stimulation, and transport rather than intrathyroidal binding is the controlling factor in making iodide available for hormonogenesis.
Extrathyroid NIS
The thyroid is not the only organ to concentrate iodine; the others endowed with this capacity are salivary glands, gastric mucosa, mammary glands, and choroid plexus. Ductal cells of the salivary glands express NIS (67) . The plasma membrane of the mammary gland epithelium contains a NIS protein with a molecular mass different from that of thyroid NIS (~75 kDa vs ~90 kDa). In the mammary gland, NIS is processed differently after translation and subjected to regulation by lactogenic stimuli (68). It has been reported that over 80% of human breast cancer samples express this symporter. As it is absent in normal non-lactating tissue, NIS may represent a marker for breast malignancy and even a possible target for radioiodine therapy (69). The thyroid, salivary glands, and gastric mucosa share a common embryologic derivation from the primitive alimentary tract and, in each of these tissues; iodide transport is inhibited by thiocyanate, perchlorate, and cardiac glycosides. TSH stimulates transport only in the thyroid. An active transport for iodide in the gastric mucosa has an obvious value because it provides iodine to the circulation for use in the thyroid. Active concentration by the breast helps transfer iodide to milk. Iodide concentration by the choroid plexus and salivary glands does not have any obvious physiologic benefit, but needs to be remembered for possible insights into pathways as yet undiscovered.
Thyroperoxidase oxidizes iodide in the presence of H2O2. TPO synthesized on polysomes is inserted in the membrane of the endoplasmic reticulum and undergoes core glycosylation. TPO is then transported to the Golgi where it is subjected to terminal glycosylation and packaged into transport vesicles along with Tg (112) (Fig. 2-6). These vesicles fuse with the apical plasma membrane in a process stimulated by TSH. TPO delivered at the apical pole of thyrocytes exposes its catalytic site with the attached heme in the thyroid follicular lumen (113). TPO activity is restricted to the apical membrane, but most of the thyroid TPO is intracellular, being located in the perinuclear part of the endoplasmic reticulum (114;115). Most of this intracellular protein is incompletely or improperly folded; it contains only high mannose-type carbohydrate units, while the membrane TPO has complex carbohydrate units. Glycosylation is essential for enzymatic activity (115). Chronic TSH stimulation increases the amount of TPO and its targetting at the apical membrane (116).
Thyroglobulin is the most abundant protein in the thyroid gland; its concentration within the follicular lumen can reach 200-300 g/L. Its main function is to provide the polypeptide backbone for synthesis and storage of thyroid hormones (180). It also offers a convenient depot for iodine storage and retrieval when external iodine availability is scarce or uneven. Neosynthesised Tg polypeptide chains entering the lumen of the rough endoplasmic reticulum (RER) are subjected to core glycosylation, dimerise and are transferred to the Golgi where they undergo terminal glycosylation (Fig. 2-7). Iodination and hormone formation of Tg occur at the apical plasma membrane-lumen boundary and the mature hormone-containing molecules are stored in the follicular lumen, where they make up the bulk of the thyroid follicle colloid content.
The step preliminary to thyroid hormone formation is the attachment of iodine to tyrosyl residues in Tg to produce MIT and DIT. This process occurs at the apical plasma membrane-follicle lumen boundary and involves H2O2, iodide, TPO, and glycosylated Tg. All rendezvous at the apical membrane to achieve Tg iodination (Fig. 2-8). First, iodide must be oxidized to an iodinating form. The final step in hormone synthesis is the coupling of two neighbouring iodotyrosyl residues to form iodothyronine
These findings suggested that iodination of lipids impairs H2O2 production and, therefore, decreases further Tg iodination. This is the most probable mechanism for the Wolff-Chaikoff effect (128).
The most important controlling factors are iodine availability and TSH. Inadequate amounts of iodine lead to inadequate thyroid hormone production, increased TSH secretion and thyroid stimulation, and goiter in an attempt to compensate. Excess iodide acutely inhibits thyroid hormone synthesis, the Wolff-Chaikoff effect (243), apparently by inhibiting H2O2 generation, and therefore, blocking Tg iodination (127). A proposed mechanism is that the excess iodide leads to the formation of 2-iodohexadecanal (255), which is endowed with an inhibitory action on H2O2 generation.
To summarize, TSH stimulates the expression of NIS, TPO, Tg and the generation of H2O2 , increases formation of T3 relative to T4, alters the priority of iodination and hormonogenesis among tyrosyls and promotes the rapid internalization of Tg by thyrocytes. These several steps are interrelated and have the net effects of increasing the amount of iodine available to the cells and of making and releasing a larger amount and a more effective type of thyroid hormone (T3).
Anti-thyroid drugs are external compounds influencing thyroid hormone synthesis. The major inhibitory drugs are the thionamides: propylthiouracil and methimazole. In the thyroid, they appear to act by competing with tyrosyl residues of Tg for oxidized iodine, at least in the rat (219). Iodotyrosyl coupling is also inhibited by these drugs and appears more sensitive to their effects than does tyrosyl iodination.
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3) Miot, Françoise, et al. “Thyroid hormone synthesis and secretion.” Thyroid disease manager (2010).
Central:Thyroid hormones (T4 and T3) are produced by the follicular cells of the thyroid gland and are regulated by TSH made by the thyrotropes of the anterior pituitary gland. The effects of T4 in vivo are mediated via T3 (T4 is converted to T3 in target tissues). T3 is 3- to 5- fold more active than T4.
Thyroxine (3,5,3′,5′-tetraiodothyronine) is produced by follicular cells of the thyroid gland. It is produced as the precursor thyroglobulin (this is not the same as TBG), which is cleaved by enzymes to produce active T4.
The steps in this process are as follows:
The Na+/I- symporter transports two sodium ions across the basement membrane of the follicular cells along with an iodine ion. This is a secondary active transporter that utilises the concentration gradient of Na+ to move I- against its concentration gradient.
I- is moved across the apical membrane into the colloid of the follicle.
Thyroperoxidase oxidises two I- to form I2. Iodide is non-reactive, and only the more reactive iodine is required for the next step.
The thyroperoxidase iodinates the tyrosyl residues of the thyroglobulin within the colloid. The thyroglobulin was synthesised in the ER of the follicular cell and secreted into the colloid.
Thyroid-stimulating hormone (TSH) released from the pituitary gland binds the TSH receptor ( a Gs protein-coupled receptor) on the basolateral membrane of the cell and stimulates the endocytosis of the colloid.
The endocytosed vesicles fuse with the lysosomes of the follicular cell. The lysosomal enzymes cleave the T4 from the iodinated thyroglobulin.
These vesicles are then exocytosed, releasing the thyroid hormones.
Thyroxine is produced by attaching iodine atoms to the ring structures of tyrosine molecules. Thyroxine (T4) contains four iodine atoms. Triiodothyronine (T3) is identical to T4, but it has one less iodine atom per molecule.
Iodide is actively absorbed from the bloodstream by a process called iodide trapping. In this process, sodium is cotransported with iodide from the basolateral side of the membrane into the cell and then concentrated in the thyroid follicles to about thirty times its concentration in the blood. Via a reaction with the enzyme thyroperoxidase, iodine is bound to tyrosine residues in the thyroglobulin molecules, forming monoiodotyrosine (MIT) and diiodotyrosine (DIT). Linking two moieties of DIT produces thyroxine. Combining one particle of MIT and one particle of DIT produces triiodothyronine.
DIT + MIT → r-T3 (biologically inactive)
MIT + DIT → triiodothyronine (usually referred to as T3)
DIT + DIT → thyroxine (referred to as T4)
Proteases digest iodinated thyroglobulin, releasing the hormones T4 and T3, the biologically active agents central to metabolic regulation.
Peripheral:
Thyroxine is believed to be a prohormone and a reservoir for the most active and main thyroid hormone T3. T4 is converted as required in the tissues by iodothyronine deiodinase. Deficiency of deiodinase can mimic an iodine deficiency. T3 is more active than T4 and is the final form of the hormone, though it is present in less quantity than T4.
4) Song, Yue, et al. “Roles of hydrogen peroxide in thyroid physiology and disease.” The Journal of Clinical Endocrinology & Metabolism 92.10 (2007): 3764-3773.
H2O2 in various cell types, and presumably in thyroid
cells, is a signal, a mitogen, a mutagen, a carcinogen, and a killer. The
To synthesize thyroid hormones, the thyrocyte takes up iodide from the blood and extracellular fluid and oxidizes it to bind it to selected tyrosines of thyroglobulin. Iodide is actively transported by the Na+/I− symporter in the cell at the basal membrane and leaked out along the electrical gradient (from the negative interior to the positive exterior) by an iodide channel at the apical membrane. Pendrin is a candidate for this role. Iodide in the follicular lumen is oxidized at the apical membrane by thyroperoxidase (TPO) using H2O2 as the other substrate. The latter originates from an H2O2-generating system whose main enzymes are the recently cloned thyroid DUOX1 and DUOX2 (45, 46). Oxidized iodide is linked covalently to tyrosines of thyroglobulin by TPO (47). The same system, by an oxidizing reaction, links covalently some iodotyrosines into iodothyronines within thyroglobulin. H2O2 is produced in large excess compared with the amounts of iodide incorporated into proteins. This may be necessary owing to the relatively high Michaelis Menten constant (Km) of TPO for H2O2 (48, 49). It is interesting that iodide leakage, i.e. presumably the iodide channel that releases iodide at the apical membrane, is acutely regulated by the same cascades and with the same timing as H2O2 generation (50).
In all species studied, iodide at high concentrations presumably through an iodinated lipid, iodohexadecanal, inhibits H2O2 generation (the Wolff-Chaikoff effect) and adenylate cyclase (62–64).
The dynamics of TPO localization in the rat thyrocyte has been beautifully demonstrated by the groups of Wollman and Ekholm (67, 68). We presume DUOX behaves similarly (Fig. 2). TPO in the thyrocyte at rest is concentrated in secretory granules just inside of the apical membrane. The iodination system is inactive there. After TSH stimulation, the granules fuse to the membrane between microvilli, allowing TPO to migrate to the microvillous membrane. It never locates on the pseudopods engulfing thyroglobulin. The main location of TPO and iodination and therefore presumably DUOX is thus in the microvillous membranes (68–70). This is supported by the demonstration by histochemistry-electron microscopy of NOX activity in the microvilli (71). Such a localization already appears, early in evolution, in the endostyle of larval amphioxus (72). In lung epithelium and in the gastrointestinal mucosa, DUOX also preferentially localizes at the brush border (51, 52).
At high concentration (above 0.1 mm), H2O2 induces apoptosis in thyroid cells, and at even higher levels (above 0.4 mm), necrosis (80–82), an effect that is potentiated by selenium deprivation and consequent GSH peroxidase depletion.
Also, iodide, at high concentration through an iodinated lipid derivative, most probably iodohexadecanal, inhibits H2O2 generation by open follicles (73, 93).
The thyroid cell contains all the biochemical systems that detoxify H2O2 in other cells, notably the selenoproteins: GSH peroxidases (cytoplasmic, plasma, and phospholipid) and thioredoxin reductases (104, 105). The concept of the TSH role in the activation of H2O2 generation and of the protective role of GSH peroxidase coupled to the hexose monophosphate pathway had already been developed in 1971 (84).
On the other hand, although the DUOX defect would suppress H2O2 generation, a TPO defect might increase H2O2 generation by decreasing its inactivation and allowing an increased generation in response to TSH.
We have repeatedly suggested that the important generation of H2O2 in thyroid cells might account for mutagenesis and the important generation of nodules in the thyroid (79, 118). This would also explain in part why more nodules are found in iodine-deficient areas.
Excess H2O2 in thyroid is neutralized in the thyrocytes by second-line mechanisms, the most efficient ones being GSH peroxidases, peroxiredoxin, and other Se-containing enzymes. Se deficiency should weaken such defenses. Indeed, low levels of serum Se have been associated with thyroid cancer
Myxedematous endemic cretinism, caused by thyroid destruction after birth, has been linked to low iodine supply in early life, leading to intense stimulation and presumably H2O2 generation, to passage from low O2 to high O2 at birth, to selenium deficiency, and thus to decreases in GSH peroxidase and thioredoxin reductase activity and to dietary thiocyanate. The experimental reproduction of this scenario in newborn rats confirms the validity of these conclusions (105, 123).
Interestingly, a similar scenario has been proposed for the physiopathology of thyroiditis (120, 124). Selenium dietary supplementation has therefore been proposed for prevention and treatment of thyroiditis and has indeed alleviated it (124, 125).
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5) Nakamura, Masao, et al. “Iodination and oxidation of thyroglobulin catalyzed by thyroid peroxidase.” Journal of Biological Chemistry 259.1 (1984): 359-364.
Thyroid peroxidase plays catalytic roles in two steps of the biosynthesis of thyroxine:
(a) the iodination of tyrosine residues of thyroglobulin and
(b) the coupling of diiodotyrosine to produce thyroxine
thyroid peroxidase oxidizes iodide to atomic iodine (I) or iodinium (I+).
The “organification of iodine,” the incorporation of iodine into thyroglobulin for the production of thyroid hormone, is nonspecific; that is, there is no TPO-bound intermediate, but iodination occurs via reactive iodine species released from TPO.[4]
The chemical reactions catalyzed by thyroid peroxidase occur on the outer apical membrane surface and are mediated by hydrogen peroxide.
6) Czarnocka, Barbara. “Thyroperoxidase, thyroglobulin, Na (+)/I (-) symporter, pendrin in thyroid autoimmunity.” Frontiers in Bioscience-Landmark 16.2 (2011): 783-802.
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Location of thyro peroxidase
7) Strum, Judy M., and Morris J. Karnovsky. “Cytochemical localization of endogenous peroxidase in thyroid follicular cells.” The Journal of cell biology 44.3 (1970): 655-666.
Peroxidase activity is found in thyroid follicular cells in the following sites: apical vesicles, associated with the external surfaces of the microvilli that project apically from the cell into the colloid.
In keeping with the radioautographic evidence of others and the postulated role of thyroid peroxidase in iodination, it is suggested that the microvillous apical cell border is the major site where iodination occurs.
Mutation in the ThYROperoxides Gene- cannot organify iodine
8) Wu, J. Y., et al. “Mutation analysis of thyroid peroxidase gene in Chinese patients with total iodide organification defect: identification of five novel mutations.” Journal of Endocrinology 172.3 (2002): 627-635.
Total iodide organification defect (TIOD), where the iodide in the thyroid gland cannot be oxidized and/or bound to the protein, is caused by a defect in the thyroid peroxidase (TPO) gene. Single strand conformation polymorphism analysis was used to screen for mutations in the TPO gene from five unrelated TIOD patients in Taiwan, and five novel mutations were detected.
9) NAGASAKA, AKIO, and HIROYOSHI HIDAKA. “Effect of antithyroid agents 6-propyl-2-thiouracil and l-methyl-2-mercaptoimidazole on human thyroid iodide peroxidase.” The Journal of Clinical Endocrinology & Metabolism 43.1 (1976): 152-158.
The mechanism of inhibition of human thyroid iodide peroxidase (TPO) by 6-propyl-2-thiouracil (PTU) and 1-methyl-2-mercaptoimidazole (MMI) used in the therapy of hyperthyroid patients was studied in vitro. MMI interacted directly with TPO and inhibited enzyme activity, rather than interacting with the product (oxidized iodide). The inhibition was irreversible with MMI, but reversible with PTU. These data may explain why MMI is a more potent inhibitor of iodination than PTU and may fit the clinical results observed when hyperthyroid patients are treated with these agents.
10) Roy, Gouriprasanna, and Govindasamy Mugesh. “Selenium analogues of antithyroid drugs–recent developments.” Chemistry & Biodiversity 5.3 (2008): 414-439.
Thyroxine (T4), the main secretory hormone of the thyroid gland, is produced on thyroglobulin by thyroid peroxidase (TPO)/H(2)O(2)/iodide system and deiodinated to its active form (T3) by a selenocysteine-containing enzyme, iodothyronine deiodinase (ID). The activation of thyroid-stimulating hormone (TSH) receptor by auto-antibodies leads to ‘hyperthyroidism’, a life-threatening disease which is treated by antithyroid drugs such as 6-propyl-2-thiouracil (PTU) and methimazole (MMI). The present review describes the biological activities of a number of S/Se derivatives bearing the methimazole pharmacophore. It is shown that the isosteric substitutions in the existing drugs lead to compounds that can effectively and reversibly inhibit the heme-containing lactoperoxidase (LPO). In contrast to methimazole, the selenium analogue, MSeI, does not interfere with the enzyme directly, but it inhibits LPO by reducing the H(2)O(2) that is required for the oxidation of the Fe-center in LPO.
11) Schmutzler, Cornelia, et al. “Selenoproteins of the thyroid gland: expression, localization and possible function of glutathione peroxidase 3.” (2007): 1053-1059.
The thyroid gland has an exceptionally high selenium content, even during selenium deficiency. At least 11 selenoproteins are expressed, which may be involved in the protection of the gland against the high amounts of H2O2 produced during thyroid hormone biosynthesis. As determined here by in situ hybridization and Northern blotting experiments, glutathione peroxidases (GPx) 1 and 4 and selenoprotein P were moderately expressed, occurring selectively in the follicular cells and in leukocytes of germinal follicles of thyroids affected by Hashimoto’s thyroiditis. Selenoprotein 15 was only marginally expressed and distributed over all cell types. GPx3 mRNA was exclusively localized to the thyrocytes, showed the highest expression levels and was down-regulated in 5 of 6 thyroid cancer samples as compared to matched normal controls. GPx3 could be extracted from thyroidal colloid by incubation with 0.5% sodium dodecyl sulfate indicating that this enzyme is (i) secreted into the follicular lumen and (ii) loosely attached to the colloidal thyroglobulin. These findings are consistent with a role of selenoproteins in the protection of the thyroid from possible damage by H2O2. Particularly, GPx3 might use excess H2O2 and catalyze the polymerization of thyroglobulin to the highly cross-linked storage form present in the colloid.
12) Schweizer, Ulrich, Jazmin Chiu, and Josef Köhrle. “Peroxides and peroxide-degrading enzymes in the thyroid.” Antioxidants & redox signaling 10.9 (2008): 1577-1592.
Iodination of thyroglobulin is the key step of thyroid hormone biosynthesis. It is catalyzed by thyroid peroxidase and occurs within the follicular space at the apical plasma membrane. Hydrogen peroxide produced by thyrocytes as an oxidant for iodide may compromise cellular and genomic integrity of the surrounding cells, unless these are sufficiently protected by peroxidases. Thus, peroxidases play two opposing roles in thyroid biology. Both aspects of peroxide biology in the thyroid are separated in space and time and respond to the different physiological states of the thyrocytes. Redox-protective peroxidases in the thyroid are peroxiredoxins, glutathione peroxidases, and catalase. Glutathione peroxidases are selenoenzymes, whereas selenium-independent peroxiredoxins are functionally linked to the selenoenzymes of the thioredoxin reductase family through their thioredoxin cofactors. Thus, selenium impacts directly and indirectly on protective enzymes in the thyroid, a link that has been supported by animal experiments and clinical observations. In view of this relationship, it is remarkable that rather little is known about selenoprotein expression and their potential functional roles in the thyroid. Moreover, selenium-dependent and -independent peroxidases have rarely been examined in the same studies. Therefore, we review the relevant literature and present expression data of both selenium-dependent and -independent peroxidases in the murine thyroid.
13) Mizukami, Y., F. Matsubara, and S. Matsukawa. “Cytochemical localization of peroxidase and hydrogen-peroxide-producing NAD (P) H-oxidase in thyroid follicular cells of propylthiouracil-treated rats.” Histochemistry 82.3 (1985): 263-268.
Abstract The distribution of endogenous peroxidase and hydrogen-peroxide-producing NAD(P)H-oxidase, which are essential enzymes for the iodination of thyroglobulin, was cytochemically determined in the thyroid follicular cells of propylthiouracil (PTU)-treated rats. Peroxidase activity was determined using the diaminobenzidine technique. The presence of NAD(P)H-oxidase was determined using H2O2 generated by the enzyme; the reaction requires NAD(P)H as a substrate and cerous ions for the formation of an electron-dense precipitate. Peroxidase activity was found in the developed rough endoplasmic reticulum (rER) and Golgi apparatus, but it was also associated with the apical plasma membrane; NAD(P)H-oxidase activity was localized on the apical plasma membrane. The presence of both enzymes on the apical plasma membrane implies that the iodination of thyroglobulin occurs at the apical surface of the follicular cell in the TSH-stimulated state which follows PTU treatment.
14) Labato, Mary Anna. “Cytochemical localization of hydrogen peroxide generating sites in the rat thyroid gland.” Tissue and Cell 17.6 (1985): 889-900.
Abstract Sites of H2O2 generation in lightly prefixed, intact thyroid follicles were studied by two cytochemical reactions: peroxidase-dependent DAB oxidation and cerium precipitation. In both cases reaction product accumulated on the apical surface of the follicle cell at the membrane-colloid interface. The former reaction was inhibited by the peroxidase inhibitor, aminotriazole; both reactions were blocked by the presence of catalase. NADH in the medium slightly increased the amount of cerium precipitation. The ferricyanide technique for oxidoreductase activity was also applied; reaction product again was associated with the apical surface. These results strongly imply that the follicle cells have a NADH oxidizing system generating H2O2 at the apical plasma membrane.
15) De la Vieja, Antonio, et al. “Molecular analysis of the sodium/iodide symporter: impact on thyroid and extrathyroid pathophysiology.” Physiological reviews 80.3 (2000): 1083-1105.
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16) Ohye, Hidemi, and Masahiro Sugawara. “Dual oxidase, hydrogen peroxide and thyroid diseases.” Experimental biology and medicine 235.4 (2010): 424-433. Ohye Hidem Dual oxidase, hydrogen peroxide and thyroid diseases Exp biology medicine 2010
The thyroid gland is a unique endocrine organ that requires hydrogen peroxide (H(2)O(2)) for thyroid hormone formation. The molecule for H(2)O(2) production in the thyroid gland has been known as dual oxidase 2 (DUOX2). Recently, NADPH oxidase 4 (NOX4), a homolog of the NOX family, was added as a new intracellular source of reactive oxygen species (ROS) in the human thyroid gland. This review focuses on the recent progress of the DUOX system and its possible contribution to human thyroid diseases. Also, we discuss human thyroid diseases related to abnormal H(2)O(2) generation. The DUOX molecule contains peroxidase-like and NADPH oxidase-like domains. Human thyroid gland also contains DUOX1 that shares 83% similarity with the DUOX2 gene. However, thyroid DUOX1 protein appears to play a minor role in H(2)O(2) production. DUOX proteins require DUOX maturation or activation factors (DUOXA1 or 2) for proper translocation of DUOX from the endoplasmic reticulum to the apical plasma membrane, where H(2)O(2) production takes place. Thyroid cells contain antioxidants to protect cells from the H(2)O(2)-mediated oxidative damage. Loss of this balance may result in thyroid cell dysfunction and thyroid diseases. Mutation of either DUOX2 or DUOXA2 gene is a newly recognized cause of hypothyroidism due to insufficient H(2)O(2) production. Papillary thyroid carcinoma, the most common thyroid cancer, is closely linked to the increased ROS production by NOX4. Hashimoto’s thyroiditis, a common autoimmune thyroid disease in women, becomes conspicuous when iodide intake increases. This phenomenon may be explained by the abnormality of iodide-induced H(2)O(2) or other ROS in susceptible individuals. Discovery of DUOX proteins and NOX4 provides us with valuable tools for a better understanding of pathophysiology of prevalent thyroid diseases.
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In dog thyroid slices thyrotropin [TSH] and carbamylcholine greatly enhance protein iodination and H2O2 generation.
17) Corvilain, Bernard, Jacqueline Van Sande, and Jacques E. Dumont. “Inhibition by iodide of iodide binding to proteins: the “Wolff-Chaikoff” effect is caused by inhibition of H2O2 generation.” Biochemical and biophysical research communications 154.3 (1988): 1287-1292.
H2O2 generation is limiting the oxidation and binding to proteins of iodide.
In dog thyroid slices thyrotropin [TSH] and carbamylcholine greatly enhance protein iodination and H2O2 generation. The action of thyrotropin is mimicked by dibutyryl cyclic AMP and forskolin which suggests that it is mediated by cyclic AMP. The action of carbamylcholine was mimicked by ionomycin and by phorbol myristate ester. This suggests that the effect of carbamylcholine is mediated by the two intracellular signals generated by the Ca++ phosphatidylinositol cascade: Ca++ and diacylglycerol. The Wolff-Chaikoff effect is the inhibition by iodide of its own organification.
In dog thyroid slices, iodide greatly inhibited H2O2 generation stimulated by thyrotropin and by carbamylcholine. Iodide decreased the production of intracellular signals induced by TSH and carbamylcholine but it also inhibited the action of probes of these intracellular signals (dibutyryl cAMP, forskolin, ionomycin, phorbol-myristate ester) on the H2O2 generating system itself. These effects were suppressed by methimazole an inhibitor of iodide oxidation.
17B) same as 116) Corvilain, Bernard, et al. “Stimulation by iodide of H2O2 generation in thyroid slices from several species.” American Journal of Physiology-Endocrinology and Metabolism 278.4 (2000): E692-E699.
In comparison with conditions in which an inhibitory effect of iodide on H2O2 generation is observed, the stimulating effect was observed for lower concentrations and for a shorter incubation time with iodide. Such a dual control of H2O2 generation by iodide has the physiological interest of promoting an efficient oxidation of iodide when the substrate is provided to a deficient gland and of avoiding excessive oxidation of iodide and thus synthesis of thyroid hormones when it is in excess. The activation of H2O2 generation may also explain the well described toxic effect of acute administration of iodide on iodine-depleted thyroids.
17C)Poncin, Sylvie, et al. “Oxidative stress in the thyroid gland: from harmlessness to hazard depending on the iodine content.” Endocrinology 149.1 (2008): 424-433.
17D Sun, Rong, et al. “Protection of Vitamin C on Oxidative Damage Caused by Long-Term Excess Iodine Exposure in Wistar Rats.” Nutrients 14.24 (2022): 5245.
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18) Markou, K., et al. “Iodine-induced hypothyroidism.” Thyroid 11.5 (2001): 501-510.
Iodine is an essential element for thyroid hormone synthesis. The thyroid gland has the capacity and holds the machinery to handle the iodine efficiently when the availability of iodine becomes scarce, as well as when iodine is available in excessive quantities. The latter situation is handled by the thyroid by acutely inhibiting the organification of iodine, the so-called acute Wolff-Chaikoff effect, by a mechanism not well understood 52 years after the original description.
It is proposed that iodopeptide(s) are formed that temporarily inhibit thyroid peroxidase (TPO) mRNA and protein synthesis and, therefore, thyroglobulin iodinations.
The Wolff-Chaikoff effect is an effective means of rejecting the large quantities of iodide and therefore preventing the thyroid from synthesizing large quantities of thyroid hormones. The acute Wolff-Chaikoff effect lasts for few a days and then, through the so-called “escape” phenomenon, the organification of intrathyroidal iodide resumes and the normal synthesis of thyroxine (T4) and triiodothyronine (T3) returns.
This is achieved by decreasing the intrathyroidal inorganic iodine concentration by down regulation of the sodium iodine symporter (NIS) and therefore permits the TPO-H2O2 system to resume normal activity.
However, in a few apparently normal individuals, in newborns and fetuses, in some patients with chronic systemic diseases, euthyroid patients with autoimmune thyroiditis, and Graves’ disease patients previously treated with radioimmunoassay (RAI), surgery or antithyroid drugs, the escape from the inhibitory effect of large doses of iodides is not achieved and clinical or subclinical hypothyroidism ensues.
Iodide-induced hypothyroidism has also been observed in patients with a history of postpartum thyroiditis, in euthyroid patients after a previous episode of subacute thyroiditis, and in patients treated with recombinant interferon-a who developed transient thyroid dysfunction during interferon-a treatment. The hypothyroidism is transient and thyroid function returns to normal in 2 to 3 weeks after iodide withdrawal, but transient T4 replacement therapy may be required in some patients. The patients who develop transient iodine-induced hypothyroidism must be followed long term thereafter because many will develop permanent primary hypothyroidism.
19) Benvenga, Salvatore, et al. “Thyroid gland: anatomy and physiology.” Encyclopedia of Endocrine Diseases 4 (2018): 382-390.
20) Harper, Mary-Ellen, and Erin L. Seifert. “Thyroid hormone effects on mitochondrial energetics.” Thyroid 18.2 (2008): 145-156.
thyroid hormones stimulate mitochondriogenesis and thereby augment cellular oxidative capacity. Thyroid hormones induce substantial modifications in mitochondrial inner membrane protein and lipid compositions. Results are consistent with the idea that thyroid hormones activate the uncoupling of oxidative phosphorylation through various mechanisms involving inner membrane proteins and lipids. Increased uncoupling appears to be responsible for some of the hypermetabolic effects of thyroid hormones. ATP synthesis and turnover reactions are also affected. There appear to be complex relationships between mitochondrial proton leak mechanisms, reactive oxygen species production, and thyroid status
21) Cioffi, Federica, et al. “Bioenergetic Aspects of Mitochondrial Actions of Thyroid Hormones.” Cells 11.6 (2022): 997.
almost all components of the respiratory chain are directly or indirectly affected by iodothyronines (see, Table 1). In some cases, the actions result in an activation of specific biochemical pathways while in others the effects would result into an increase in mRNA or protein levels of specific components of respiratory chain.
22) Szanto, Ildiko, Marc Pusztaszeri, and Maria Mavromati. “H2O2 metabolism in normal thyroid cells and in thyroid tumorigenesis: focus on NADPH oxidases.” Antioxidants 8.5 (2019): 126.
23) Ghaddhab, Chiraz, et al. “Factors contributing to the resistance of the thyrocyte to hydrogen peroxide.” Molecular and cellular endocrinology 481 (2019): 62-70.
24) Fortunato, Rodrigo Soares, et al. “Functional consequences of dual oxidase-thyroperoxidase interaction at the plasma membrane.” The Journal of Clinical Endocrinology & Metabolism 95.12 (2010): 5403-5411.
25) Lanzolla, Giulia, Michele Marinò, and Claudio Marcocci. “Selenium in the treatment of Graves’ hyperthyroidism and eye disease.” Frontiers in Endocrinology 11 (2021): 608428.
26) Ruggeri, Rosaria M., et al. “Selenium exerts protective effects against oxidative stress and cell damage in human thyrocytes and fibroblasts.” Endocrine 68.1 (2020): 151-162.
27) Schomburg, Lutz, and Josef Köhrle. “On the importance of selenium and iodine metabolism for thyroid hormone biosynthesis and human health.” Molecular nutrition & food research 52.11 (2008): 1235-1246.
28) Brix, Klaudia, et al. “Thyroglobulin storage, processing and degradation for thyroid hormone liberation.” The Thyroid and Its Diseases. Springer, Cham, 2019. 25-48.
29) Fallahi, Poupak, et al. “Myo-inositol in autoimmune thyroiditis, and hypothyroidism.” Reviews in Endocrine and Metabolic Disorders 19.4 (2018): 349-354.
30) Ferrari, Silvia Martina, et al. “The protective effect of myo-inositol on human thyrocytes.” Reviews in Endocrine and Metabolic Disorders 19.4 (2018): 355-362.
31) Benvenga, Salvatore, et al. “The role of inositol in thyroid physiology and in subclinical hypothyroidism management.” Frontiers in Endocrinology (2021): 458.
32) Portulano, Carla, Monika Paroder-Belenitsky, and Nancy Carrasco. “The Na+/I− symporter (NIS): mechanism and medical impact.” Endocrine reviews 35.1 (2014): 106-149.
The Na+/I− symporter (NIS) is the plasma membrane glycoprotein that mediates active I− transport in the thyroid and other tissues, such as salivary glands, stomach, lactating breast, and small intestine.
Since the cloning of NIS in 1996, NIS research has become a major field of inquiry
The significance of the thyroid gland for human health is difficult to overstate, given the wide-ranging effects of the thyroid hormones (THs) on prenatal and early development as well as on intermediary metabolism at all stages of life. Thyroid function, in turn, depends on an adequate supply of iodide (I−), an essential constituent of the THs. I− reaches the thyroid via a highly specialized I−-concentrating system, the Na+/I− symporter (NIS), the key plasma membrane transport protein located on the basolateral surface of thyrocytes.
Inadequate dietary I− intake results in hypothyroidism and goiter at all ages. Mild ID also has a significant impact on reproductive health, increasing miscarriages and stillbirths, and is the most common cause of reproductive failure worldwide (9). In addition, it is associated with increased perinatal mortality in the population (9, 10).
Insufficient I− intake in critical windows of development during fetal life and childhood results in the most dramatic manifestation of ID disorders (IDDs), ie, neurological cretinism, characterized by severe and permanent growth and cognitive impairment. The TH T4, which includes 4 iodine atoms in its structure (Figure 1 and Section III.B.), has a profound effect on brain development; it is required for myelination, somatogenesis, neuronal differentiation, and formation of neural processes, and its function is critical for proper development of the cerebral cortex, cochlea, and basal ganglia during the first trimester of gestation and for brain growth and differentiation in the third trimester
More subtly, suboptimal I− intake is a cause of reduced mental ability, lower school and work performance, and loss of an estimated 13.5 intelligence quotient (IQ) points at a population level (11) and intellectual potential (for a brief overview, see Refs. 11–13). The urinary I− (UI) concentration (normal value >100 μg/L) is a significant indicator of thyroid function and the most useful measure of I− status of a given population. Even in I−-sufficient countries, suboptimal I− intake still affects a significant number of people. In the United States, ∼8% of the population has a UI <50 μg/L. This percentage increases in women of reproductive age to 14.8%, and ∼11% of pregnant women have also been reported to have a UI <50 μg/L, both numbers higher than in previous years (14, 15). Strikingly, ID is still the most common cause of preventable brain damage after starvation.
NIS (5) (Figure 2), which is located on the basolateral membrane of thyroid epithelial cells
As indicated above, THs thyroid hormones are required for the development and maturation of the central nervous system (CNS), lungs, and skeleton (21, 24–26). They are master regulators of cell metabolism (lipid, protein, and carbohydrate anabolism and catabolism). Thus, their deficiency or excess perturbs homeostasis and leads to disease.
The functional units of the mature thyroid gland are the thyroid follicles, which are enclosed by a basement membrane. Histologically, the spherical follicles are lined by a monolayer of polarized epithelial cells, with the basolateral surface facing the bloodstream and the apical surface the follicular lumen (Figure 1). The lumen is filled with colloid, which is mainly composed of thyroglobulin (Tg), a protein present at a very high concentration (∼100–750 mg/mL) (27) that serves as the backbone for the THs. I− is covalently incorporated into Tg, a process called I− organification; iodinated Tg is the precursor and storage form of the THs, which are released into the bloodstream upon stimulation by TSH (see Section III.B. for more details). The thyroid is unique among endocrine glands in that its hormonal products are stored extracellularly in the colloid.
One candidate to be an apical I− transporter is pendrin, In conclusion, although various candidate proteins that may mediate I− translocation across the apical membrane of thyrocytes have been proposed, at the present time, the identity of the I− apical translocation pathway remains poorly understood.
The oxidative power for I− organification is provided by H2O2 generated by dual oxidase 2 (Duox2), a multispanning membrane protein belonging to the family of the nicotinamide adenine dinucleotide phosphate-dependent flavin adenine dinucleotide oxidases. The name dual oxidase stems from the presence of a lumen-facing domain that exhibits >45% amino acid similarity with TPO, with putative and controversial peroxidase activity (44–46). Interestingly, Duox2 shares 47% amino acid similarity to gp91phox (Nox2), which is responsible for the respiratory burst and the production of reactive oxygen species (ROS) in neutrophils (45). In contrast to most of the proteins involved in TH biosynthesis, Duox2 expression and activity do not appear to be regulated by TSH.
When TSH serum levels increase, all steps of TH biosynthesis and release are stimulated, including Tg internalization.
Extrathyroidal accumulation of I− has been reported in the salivary glands, gastric mucosa, small intestine, lactating mammary gland, choroid plexus, and the ciliary body of the eye (127, 186). However, in contrast to the thyroid, I− is not organified and I− uptake is not regulated by TSH in these tissues
In addition to NIS, TSH also upregulates expression of TPO and Tg as well as iodinated Tg endocytosis. Levy et al (113) have shown that thyroid NIS protein expression is upregulated by TSH in vivo.
In addition to TSH, I− itself is a major regulator of I− accumulation in the thyroid. The ability to block thyroid function with high I− was described by Plummer (219) as early as 1923. In 1948, Wolff and Chaikoff (220) reported that high plasma I− levels blocked I− organification in rat thyroid in vivo. The ability of high doses of I− to decrease thyroid function became known as the Wolff-Chaikoff effect (Figure 11A). However, this inhibitory effect of I− lasts ∼2 days in the presence of high plasma I−, after which TH biosynthesis is restored (221). This adaptation to persistent high I− was termed the escape from the Wolff-Chaikoff effect (Figure 11B).
What is the mechanism for the escape/decrease in I− transport? In 1986, it was reported that I− preincubation suppressed I− uptake in FRTL-5 cells in a time- and dose-dependent manner, an effect preventable with methimazole (MMI), which inhibits I− organification (223). Inhibition by MMI suggested that the Wolff-Chaikoff effect was mediated by an intracellular iodinated compound. Consistent with the observed reduction in I− transport, later studies demonstrated an I−-induced reduction in NIS and TPO mRNA in dog thyroid (224) and NIS mRNA and I− transport in FRTL-5 cells (225). A decrease in NIS mRNA and protein as well was demonstrated 24 hours after incubation with high I−, but no correlation was made with NIS activity (226).
High concentrations of I−, presumably in the form of iodolipids, inhibit the generation of H2O2 (229–231). Interestingly, iodolipids have been found in rat, horse, dog, and human thyroid. These iodolipids inhibit Duox and adenylyl cyclase, suggesting that they are involved in regulatory actions of I− in the thyroid (232, 233).
33) Serrano-Nascimento, Caroline, et al. “The acute inhibitory effect of iodide excess on sodium/iodide symporter expression and activity involves the PI3K/Akt signaling pathway.” Endocrinology 155.3 (2014): 1145-1156.
34) Eleftheriadou, Anna-Maria, et al. “Re-visiting autoimmunity to sodium-iodide symporter and pendrin in thyroid disease.” European Journal of Endocrinology 183.6 (2020): 571-580..
35) Ihnatowicz, Paulina, Paweł Wątor, and Małgorzata Drywień. “Supplementation in Autoimmune Thyroid Hashimoto’s Disease. Vitamin D and Selenium.” J. Food Nutr. Res 7 (2019): 584-591.
36) Tsatsoulis, Agathocles. “The role of iodine vs selenium on the rising trend of autoimmune thyroiditis in iodine sufficient countries-an opinion article.” Open Acc J Thy Res 2.1 (2018): 12-14.
37) Rostami, Rahim, et al. “Serum selenium status and its interrelationship with serum biomarkers of thyroid function and antioxidant defense in Hashimoto’s thyroiditis.” Antioxidants 9.11 (2020): 1070.\\
38) Tian, Xun, et al. “Selenium supplementation may decrease thyroid peroxidase antibody titer via reducing oxidative stress in euthyroid patients with autoimmune thyroiditis.” International Journal of Endocrinology 2020 (2020).
39) Vasiliu, Ioana, et al. “Experimental induced autoimmune thyroiditis in wistar rats: possible protective role of selenium.” Endocrine Abstracts. Vol. 70. Bioscientifica, 2020.
40) Dashdamirova, Gulnara, et al. “Pathogenic Mechanisms of Autoimmune Thyroid Disease.” Int J Med Sci Health Res 6 (2022): 26-33
41) Moncayo, Roy, Helga Moncayo, and Juliana Reisenzahn. “Global view on the pathogenesis of benign thyroid disease based on historical, experimental, biochemical and genetic data, identifying the role of magnesium, selenium, coenzyme Q10 and iron in the context of the unfolded protein response and protein quality control of thyroglobulin.” Journal of Translational Genetics and Genomics 4.4 (2020): 356-383.
42) Xu, Bin, et al. “A pilot study on the beneficial effects of additional selenium supplementation to methimazole for treating patients with Graves’ disease.” Turkish journal of medical sciences 49.3 (2019): 715-722.
43) Zhang, Xiao-Hong, Gao-Pin Yuan, and Ting-Li Chen. “Clinical effect of methimazole combined with selenium in the treatment of toxic diffuse goiter in children.” World Journal of Clinical Cases 10.4 (2022): 1190.
44) Gallo, Daniela, et al. “Add-on effect of selenium and vitamin D combined supplementation in early control of Graves’ disease hyperthyroidism during methimazole treatment.” Frontiers in Endocrinology 13 (2022).
45) Barbaro, Daniele, Beatrice Orrù, and Vittorio Unfer. “Iodine and myo-inositol: a novel promising combination for iodine deficiency.” Frontiers in Endocrinology 10 (2019): 457.
Myo-inositol, a carbocyclic polyol, regulates the generation of hydrogen peroxide (H2O2) in thyrocytes, crucial for iodine organification and thyroid hormone biosynthesis.
Either in vitro and in vivo iodide main roles are:
(1) decreasing the response of TSH;
(2) inhibiting its own oxidation (the Wolff-Chaikoff effect);
(3) reducing its trapping after a delay;
(4) at high levels, inhibiting thyroid hormone secretion (28).
The Wolff–Chaikoff effect is an autoregulatory phenomenon that prevents the thyroid gland from synthesizing and releasing large quantities of thyroid hormones (19, 29) through the inhibition of organification when inorganic iodide levels in thyrocytes are too high (30, 31). The mechanism responsible for such effect is still unknown, but it may be ascribable to the inhibitory effect of iodide on TPO or other enzymes (19). Indeed, iodide is able to block the cyclic adenosine monophosphate cascade and the Ca2+-phosphatidylinositol 4, 5-bisphosphate (PIP2) cascade in thyrocytes and to induce H2O2 generation (28). H2O2 is crucial for the TPO-catalyzed thyroid hormone formation (32). In 1993, Chen at al. demonstrated in vitro that H2O2 specifically regulates iodide transport and organification in a dose-dependent manner (33). High H2O2 concentrations inhibit these functions and can be detrimental to the thyroid, although protection mechanisms prevents damages to the thyrocytes under physiological conditions (34). Intrathyroidal H2O2 generation, first reported about 50 years ago (35), and iodination are both stimulated by TSH. H2O2 can regulate iodination either directly, as a substrate of TPO, or indirectly by regulating the activity of TPO when iodide and Tg concentrations are kept constant.
As precursor also of the second messenger phosphoinositide, myo-inositol is involved in cell signaling and regulates the activities of hormones such as TSH, follicle-stimulating hormone (FSH) and insulin (37–39).
It recently proved to be a very efficacious and safe treatment for subclinical hypothyroid patients with Autoimmune Thyroiditis (40–46). In 2013, Nordio and Pajalich demonstrated that treatment with myo-inositol plus selenium for 6 months in patients with subclinical hypothyroidism reduced significantly TSH concentrations by 31% (4.4 ± 0.9 mIU/mL vs. 3.1 ± 0.6 mIU/mL), compared to the control group treated only with selenium.
Interestingly, myo-inositol helps thyroid-hormone-producing cells to become more efficient and faster at building T4 (32, 41). This might be ascribable to a higher availability of iodine, whose organification is boosted by myo-inositol action. Indeed, myo-inositol is involved in one of the first steps of thyroid hormone production and modulates the H2O2-mediated iodination through the phospholipase C-dependent inositol phosphate Ca2+/diacylglycerol pathway, resulting in increased H2O2 generation (Figure 1).
Differently, the cAMP cascade, induced by the TSH activity (through the TSH receptor activation), is more involved in cell growth and differentiation, and in thyroid hormones secretion.
As myo-inositol plays a crucial role in the regulation of iodine organification, supplementation may promote faster recovery from ID. Indeed, H2O2 generated under the stimulus of myo-inositol is available for iodine incorporation inside the thyroid (33, 48). Such activity makes myo-inositol very appealing as a novel molecule to increase iodine availability.
To date, endocrinologists and gynecologists recommend taking myo-inositol for several benefits in different pathologies, from thyroid diseases to polycystic ovary syndrome and gestational diabetes. Due to its action in regulating iodine organification and thyroid hormone biosynthesis, myo-inositol supplementation along with iodine may improve thyroid functionality and possibly lead to a faster recovery from ID. … When iodine intake is insufficient, the T3:T4 ratio increases along with the decrease of T4 production,
46) Rus-Hrincu, Florentina, Alexandra Totan, and A. M. A. Stanescu. “DUOX2, a new player on the scene of thyroid hormones.” Practica Medicala 14.3 (2019): 67.
48) Eng, Peter HK, et al. “Escape from the acute Wolff-Chaikoff effect is associated with a decrease in thyroid sodium/iodide symporter messenger ribonucleic acid and protein.” Endocrinology 140.8 (1999): 3404-3410.
49) Bogusławska, Joanna, et al. “Cellular and molecular basis of thyroid autoimmunity.” European Thyroid Journal 11.1 (2022).
TPO autoantibodies (TPOAb) occur in almost all HT patients and approximately 75% of individuals with GD, whereas their positivity in euthyroid patients may be the risk factor for future thyroid disorders (15).
The immune response to iodide transporters, sodium iodide symporter (NIS) and pendrin, has been widely discussed (24, 25
In contrast to TPO and Tg, TSHR is widely expressed in extrathyroidal tissues and cells, including lymphocytes, adipose tissue, and fibroblasts. In consequence, the presence of TSHR antibodies contributes to the extrathyroidal GD manifestations, such as Graves’ ophthalmopathy (GO), Graves’ dermopathy, or GD-associated thymus hyperplasia (1, 6). TSHR autoantigen is presented by macrophages and B cells recruited to the orbit, which leads to T cell activation (5). In turn, activated T cells initiate immunological attack to the orbital fibroblasts expressing TSHR
50) Roy, Gouriprasanna, and G. Mugesh. “Thyroid hormone synthesis and anti-thyroid drugs: A bioinorganic chemistry approach.” Journal of Chemical Sciences 118.6 (2006): 619-625.
The most widely used anti-thyroid drug methimazole (MMI) inhibits the production of thyroid hormones by irreversibly inactivating the enzyme TPO.
51) Lane, Laura C., Claire Louise Wood, and Tim Cheetham. “Graves’ disease: moving forwards.” Archives of Disease in Childhood (2022).
Thionamides block thyroid hormone synthesis by inhibiting the TPO-mediated iodination of tyrosine residues in thyroglobulin, a vital step in the synthesis of thyroxine
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52) Sugawara, M., Y. Sugawara, and K. Wen. “Methimazole and propylthiouracil increase cellular thyroid peroxidase activity and thyroid peroxidase mRNA in cultured porcine thyroid follicles.” Thyroid: Official Journal of the American Thyroid Association 9.5 (1999): 513-518.
In conclusion, MMI and PTU at therapeutic concentrations can increase TPO mRNA and cellular TPO activity, although the 2 drugs inhibit the TPO-H2O2-mediated catalytic reaction.
Explains the 10% rebound phenomenon when switching from MMI to Iodine in Pregnancy Graves
53) Landex, N. L., J. Thomsen, and L. Kayser. “Methimazole increases H2O2 toxicity in human thyroid epithelial cells.” Acta histochemica 108.6 (2006): 431-439.
54) Roy, Gouriprasanna, and G. Mugesh. “Thyroid hormone synthesis and anti-thyroid drugs: A bioinorganic chemistry approach.” Journal of Chemical Sciences 118.6 (2006): 619-625.
55) Manna, Debasish, Gouriprasanna Roy, and Govindasamy Mugesh. “Antithyroid drugs and their analogues: synthesis, structure, and mechanism of action.” Accounts of chemical research 46.11 (2013): 2706-2715.
The prohormone thyroxine (T4) is synthesized on thyroglobulin by thyroid peroxidase (TPO), a heme enzyme that uses iodide and hydrogen peroxide to perform iodination and phenolic coupling reactions. The monodeiodination of T4 to 3,3′,5-triiodothyronine (T3) by selenium-containing deiodinases (ID-1, ID-2) is a key step in the activation of thyroid hormones. The type 3 deiodinase (ID-3) catalyzes the deactivation of thyroid hormone in a process that removes iodine selectively from the tyrosyl ring of T4 to produce 3,3′,5′-triiodothyronine (rT3).
56) Ortiz de Montellano, Paul R. “Hemes in biology.” Wiley encyclopedia of chemical biology (2007): 1-10.
Heme is a ubiquitous prosthetic group in proteins with oxidation- reduction functions. The proteins that incorporate heme as an essential component include respiratory proteins, such as cytochrome c and cytochrome c oxidase. The oxygen carriers myoglobin and hemoglobin are members of a second class of hemoproteins that have some relationship to gas sensor hemoproteins, as represented for NO by guanylate cyclase; for O2 by Hap1, FixL, and HemAT; and for CO by CooA. The largest class of hemoproteins is made up of catalytic enzymes and includes the cytochromes P450, peroxidases, peroxygenases, catalases, NO synthases, prostaglandin synthases, and thromboxane/prostacyclin synthases.
TPO is a heme protein containing a porphyrin ring
57) de Castro, Guilherme Vieira, et al. “Synthesis of Analogues of Thyroid Hormones: Nuclear Receptor Modulators.” Orbital: The Electronic Journal of Chemistry 7.3 (2015): 282-291.
58) Pavelka, S. “Metabolism of bromide and its interference with the metabolism of iodine.” Physiological Research 53 (2004): S81-90.
59) Kalderon, Albert E., and Hendrik A. Bogaars. “Immune complex deposits in Graves’ disease and Hashimoto’s thyroiditis.” The
60) Matsuta, Morimasa. “Immunohistochemical and electron microscopic studies on Hashimoto’s thyroiditis.” Pathology International 32.1 (1982): 41-56.
61) Fujiwara, Hiroshi, et al. “Immune complex deposits in thyroid glands of patients with Graves’ disease: II. Anti-thyroglobulin and anti-microsome activities of γ-globulin eluted from thyroid homogenates.” Clinical Immunology and Immunopathology 19.1 (1981): 109-117.
62) Solis-S, Juan C., et al. “Inhibition of intrathyroidal dehalogenation by iodide.” Journal of endocrinology 208.1 (2011): 89.
63) Dugrillon, A. “Iodolactones and iodoaldehydes—mediators of iodine in thyroid autoregulation.” Experimental and clinical endocrinology & diabetes 104.S 04 (1996): 41-45.
64) Laurent, Eric, et al. “Unlike thyrotropin, thyroid-stimulating antibodies do not activate phospholipase C in human thyroid slices.” The Journal of clinical investigation 87.5 (1991): 1634-1642.
65) Freitas Ferreira, Andrea C., et al. “Thyroid Ca2+/NADPH‐dependent H2O2 generation is partially inhibited by propylthiouracil and methimazole.” European Journal of Biochemistry 270.11 (2003): 2363-2368.
66) Molnár, Ildikó, et al. “Effect of antithyroid drugs on the occurrence of antibodies against type 2 deiodinase (DIO2), which are involved in hyperthyroid Graves’ disease influencing the therapeutic efficacy.” Clinical and Experimental Medicine 19.2 (2019): 245-254.
67) Molnár, Ildikó. “Relationship between autoantibodies against type 2 deiodinase and antithyroid autoantibodies in Graves’ disease using antithyroid drug therapy.” Medical Research Archives 10.12 (2022).
68) Morand, Stanislas, et al. “Effect of iodide on nicotinamide adenine dinucleotide phosphate oxidase activity and Duox2 protein expression in isolated porcine thyroid follicles.” Endocrinology 144.4 (2003): 1241-1248.
69) Cardoso, Luciene C., et al. “Ca2+/nicotinamide adenine dinucleotide phosphate-dependent H2O2 generation is inhibited by iodide in human thyroids.” The Journal of Clinical Endocrinology & Metabolism 86.9 (2001): 4339-4343.
70) Abraham, Guy E. “The safe and effective implementation of orthoiodosupplementation in medical practice.” The Original Internist 11.1 (2004): 17-36.
71) Suzuki, Hiroshi, Hiroshi Sano, and Hisashi Fukuzaki. “Decreased cytosolic free calcium concentration in lymphocytes of magnesium-supplemented DOCA-salt hypertensive rats.” Clinical and Experimental Hypertension. Part A: Theory and Practice 11.3 (1989): 487-500.
72) Yang, Ying, et al. “Magnesium deficiency enhances hydrogen peroxide production and oxidative damage in chick embryo hepatocyte in vitro.” Biometals 19.1 (2006): 71-81.
73) Betterle, Corrado, and Renato Zanchetta. “The immunofluorescence techniques in the diagnosis of endocrine autoimmune diseases.” Autoimmunity Highlights 3.2 (2012): 67-78.
74) Wolff, Jirj, et al. “The temporary nature of the inhibitory action of excess iodide on organic iodine synthesis in the normal thyroid.” Endocrinology 45.5 (1949): 504-513.
75) Leoni, Suzana G., et al. “Regulation of Thyroid Oxidative State by Thioredoxin Reductase Has a Crucial Role in Thyroid Responses to Iodide Excess.” MOLECULAR ENDOCRINOLOGY 25.11 (2011): 1924.
76) Murphy, Robyn, Caroline Turek, and Leigh Arseneau. “The role of iodine deficiency and subsequent repletion in autoimmune thyroid disease and thyroid cancer.” Journal of Restorative Medicine 5.1 (2016): 32.
The sodium-iodide symporter (NIS) transports iodide (I−) into the thyrocyte to be organified to iodine (I2) and bound to thyroglobulin (TG) by thyroid peroxidase (TPO) and hydrogen peroxide (H2O2) (Figure 1). The iodine–thyroglobulin complexes then combine to form thyroid hormones. The nicotinamide adenine dinucleotide phosphate (NADPH) oxidase system is upregulated by intracellular calcium to generate H2O2, a reactive oxygen species (ROS). To prevent excess H2O2, iodolactones negatively inhibit NADPH oxidase, and glutathione peroxidase degrades H2O212–14 (Figure 1). In iodine deficiency, the loss of negative feedback on H2O2 production has been implicated in thyroid dysfunction and as a possible mechanism in the generation of autoantibodies.
The import of iodide (I−) by the sodium-iodide symporter (NIS) into the thyrocyte and the subsequent organification of I− to form iodine (I2).15 I2 is then is bound to thyroglobulin (TG) to form thyroid hormones, thyroxine (T4), and triiodothyronine (T3). The organification process is catalyzed by thyroid peroxidase (TPO) and hydrogen peroxide (H2O2), produced by the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase system. Iodinated lipids provide negative feedback on NADPH oxidase to reduce H2O2 and oxidative damage to the thyrocyte.
Studies suggest that when iodine intake is above 1.5 mg daily, TPO synthesizes iodolactones.16 TPO facilitates the iodination of arachidonic acid and other polyunsaturated fatty acids to produce iodolactones.17 Research shows that measurable concentrations of δ-iodolactone are found in the thyroid tissue, prevent excess iodide uptake and the generation of H2O2 by NADPH oxidase, and regulate thyroid function.18
However, in iodine deficiency, iodolactones are obsolete, and the thyroid gland is susceptible to oxidative damage and loss of cell-cycle control.
In chronic iodine deficiency, compensatory mechanisms fail to maintain iodine concentration, and the thyroid is susceptible to damage. As iodine concentrations decrease, the pituitary gland secretes thyroid-stimulating hormone (TSH) to induce NIS expression.18 TSH remains high, and the NIS fails to shut down, predisposing the gland to excess iodine during supplementation, a phenomenon found in individuals with preexisting autoimmune thyroiditis (AIT).4 In addition, thyroid activity remains high, leading to hyperplasia and goiter.12 Researchers in cross-sectional studies reported that patients with nodular goiters often have an increase in thyroid antibodies, which is more common in iodine deficiency.4,21 These observations suggest that immune stimulation to thyroid proteins occurs during iodine deficiency and may be a causative factor in the development of thyroid autoimmunity.
In iodine deficiency, the thyroid is susceptible to oxidative damage. As TSH stimulates NIS and TPO activity, low levels of iodinated lipids with high cytosolic free calcium allow for excess H2O2 to be produced.16 Iodine alone fails to initiate immune activation; rather, experimental studies show that iodine in the presence of inflammatory cytokines augments immune function. In an in vitro study, researchers observed increased ICAM-1 mRNA expression in the presence of potassium iodide only when low-dose IFN-γ was added.23,27 In the presence of excess H2O2, oxidative damage to TPO and TG cells leads to antigen presentation and lymphocytic infiltration, which facilitates the production of anti-TPO and anti-TG antibodies.16,28 This mechanism may account for the prevalence of AIT upon iodine supplementation through USI programs.
The lowest concentrations of iodine are found in individuals with thyroid autoimmunity. In 13 patients with overt symptoms of hypothyroidism and AIT, an average of 2.3 mg of iodine was found in the thyroid, as compared with 10 mg in healthy subjects.16 On the basis of data derived from the National Health and Nutrition Examination Survey, although iodine intake in the United States continues to decrease, the prevalence of AIT continues to increase.21,30,31 This correlation suggests that iodine deficiency may be a causative factor in patients with AIT.
Na+/I− symporter
77) Elliyanti, Aisyah, et al. “An Iodine Treatments Effect on Cell Proliferation Rates of Breast Cancer Cell Lines; In Vitro Study.” Open Access Macedonian Journal of Medical Sciences 8.B (2020): 1064-1070.
78) Zuckier, Lionel S., et al. “The endogenous mammary gland Na+/I− symporter may mediate effective radioiodide therapy in breast cancer.” Journal of Nuclear Medicine 42.6 (2001): 987-987.
79) Ravera, Silvia, et al. “The sodium/iodide symporter (NIS): molecular physiology and preclinical and clinical applications.” Annual review of physiology 79 (2017): 261.
Functional NIS is found in several extrathyroidal tissues, such as the salivary glands, stomach, and lactating breast, as well as in primary and metastatic breast cancers (7, 8). The latter findings have raised the possibility that NIS-mediated 131I− treatment may be effective in breast cancer.
One of the most remarkable properties of NIS is that it transports different substrates with different stoichiometries. NIS transports I−, thiocynate (SCN−) and chlorate (ClO3−) with a 2 Na+:1 anion electrogenic stoichiometry. In contrast, NIS transports perrhenate (ReO4−) and perchlorate (ClO4−) with a 1 Na+:1 anion electroneutral stochiometry (9). The anion ClO4−, an environmental pollutant previously known only as a competitive inhibitor of NIS, is actively transported by NIS as a substrate (10). Furthermore, NIS is increasingly used as a highly effective reporter gene for imaging techniques (11, 12).
TSH is the primary regulator of NIS in the thyroid at both the transcriptional and post-transcriptional levels (25, 31–35) (see 36 for a recent review). Another regulator of NIS function is I− itself. Wolff & Chaikoff (37) showed that when I− reaches a critical high concentration in the plasma, TH biosynthesis decreases (a.k.a. the Wolff-Chaikoff effect). However, there is an “escape” from this acute Wolff-Chaikoff effect that restores normal TH biosynthesis even in the continued presence of high plasma I− concentrations (38) (see 14 for a recent review). At the molecular level, excess I− may have a deleterious effect on the thyroid by modifying NIS mRNA stability and increasing the production of reactive oxygen species (39, 40).
NIS function is traditionally associated with the thyroid. However, active I− transport has also been also demonstrated in other organs (Figures 2 and and3),3), including the lacrimal sac and nasolacrimal duct, salivary glands, choroid plexus, stomach, intestine, and lactating breast (14). Reverse transcription polymerase chain reaction (RT-PCR) and immunodetection have further uncovered NIS expression in the kidney, placenta, and ovary (49–53)
NIS is expressed in the normal breast only late in pregnancy and during lactation
NIS is also expressed in mammary gland tumors in animal models of breast cancer and, more importantly, in human breast cancer samples. As many as 87% of the breast tumor samples analyzed by Tazebay et al. (7) tested positive for NIS expression, in contrast with 0% of samples of normal nonlactating breast tissue
80) Tazebay, Uygar H., et al. “The mammary gland iodide transporter is expressed during lactation and in breast cancer.” Nature medicine 6.8 (2000): 871-878.
81) Zuckier, Lionel S., et al. “The endogenous mammary gland Na+/I− symporter may mediate effective radioiodide therapy in breast cancer.” Journal of Nuclear Medicine 42.6 (2001): 987-987.
82) Yao, Chen, et al. “Effect of sodium/iodide symporter (NIS)-mediated radioiodine therapy on estrogen receptor-negative breast cancer.” Oncology Reports 34.1 (2015): 59-66.
83) Elliyanti, Aisyah, et al. “Analysis natrium iodide symporter expression in breast cancer subtypes for radioiodine therapy response.” Nuclear Medicine and Molecular Imaging 54.1 (2020): 35-42.
84) Nava-Villalba, Mario, and Carmen Aceves. “6-iodolactone, key mediator of antitumoral properties of iodine.” Prostaglandins & other lipid mediators 112 (2014): 27-33.
85) Aceves, Carmen, et al. “Molecular iodine has extrathyroidal effects as an antioxidant, differentiator, and immunomodulator.” International Journal of Molecular Sciences 22.3 (2021): 1228.
86) Dunn, John T., and Ann D. Dunn. “Update on intrathyroidal iodine metabolism.” Thyroid 11.5 (2001): 407-414.
87) Rossich, Luciano E., et al. “Effects of 2-iodohexadecanal in the physiology of thyroid cells.” Molecular and cellular endocrinology 437 (2016): 292-301.
88) Köhrle, J. “Thyrotropin (TSH) action on thyroid hormone deiodination and secretion: one aspect of thyrotropin regulation of thyroid cell biology.” Hormone and Metabolic research. Supplement Series 23 (1990): 18-28.
89) Dohan, Orsolya, et al. “The sodium/iodide symporter (NIS): characterization, regulation, and medical significance.” Endocrine reviews 24.1 (2003): 48-77.
90) Ravera, Silvia, et al. “The sodium/iodide symporter (NIS): molecular physiology and preclinical and clinical applications.” Annual review of physiology 79 (2017): 261.
91) Riesco-Eizaguirre, Garcilaso, Pilar Santisteban, and Antonio de la Vieja. “The complex regulation of NIS expression and activity in thyroid and extrathyroidal tissues.” (2021).
92) Martín, Mariano, et al. “Implications of Na+/I-symporter transport to the plasma membrane for thyroid hormonogenesis and radioiodide therapy.” Journal of the Endocrine Society 3.1 (2019): 222-234.
93) Micali, Salvatore, et al. “Sodium iodide symporter (NIS) in extrathyroidal malignancies: focus on breast and urological cancer.” BMC cancer 14.1 (2014): 1-12.
94) Shiozaki, Atsushi, et al. “Functional analysis and clinical significance of sodium iodide symporter expression in gastric cancer.” Gastric Cancer 22.3 (2019): 473-485.
95) Dwyer, R. M., et al. “Sodium iodide symporter-mediated radioiodide imaging and therapy of ovarian tumor xenografts in mice.” Gene Therapy 13.1 (2006): 60-66.
96) Nicola, Juan Pablo, et al. “The Na+/I− symporter mediates active iodide uptake in the intestine.” American Journal of Physiology-Cell Physiology 296.4 (2009): C654-C662.
97) Spitzweg, Christine, et al. “Expression of the sodium iodide symporter in human kidney.” Kidney international 59.3 (2001): 1013-1023.
99) Ross, Douglas S. “Thionamides in the treatment of Graves’ disease.” UpToDate 9 (2001): 1.
100) Ross, Douglas S. “Graves’ hyperthyroidism in nonpregnant adults: Overview of treatment.” UpToDate (2020).
101) Takeuchi, Keisuke, et al. “Significance of iodide-perchlorate discharge test for detection of iodine organification defect of the thyroid.” The Journal of Clinical Endocrinology & Metabolism 31.2 (1970): 144-146.
Perchlorate tests were performed after oral administration of radioiodine without carrier iodide (conventional perchlorate test) or with 500 μg of 127I (iodide-perchlorate test). In neither test was there a significant discharge of 131I in control subjects. In subjects treated with l-methyl-2-mercaptoimidazole positive discharge was observed with the iodide-perchlorate test, but not with conventional perchlorate test. Of 8 patients with Hashimoto’s thyroiditis, only 2 showed positive discharge tests by the conventional perchlorate test, whereas the iodide-perchlorate test was positive in all these patients.
102) Tajiri, Junichi, et al. “Studies of hypothyroidism in patients with high iodine intake.” The Journal of Clinical Endocrinology & Metabolism 63.2 (1986): 412-417.
Twenty-two patients with spontaneously occurring primary hypothyroidism were studied to evaluate the spontaneous reversibility of the hypothyroid state. Twelve (54.5%) became euthyroid after restriction of iodine intake for 3 weeks (reversible type). In the remaining 10 patients, thyroid function did not improve with restriction of iodine alone, and thus, replacement therapy was required, (irreversible type). In the reversible type, 1) radioactive iodine uptake after 1 week of restricted iodine intake was higher than in the irreversible type [50.0 +/- 12.2% (+/- SD) vs. 4.3 +/- 3.2%; P less than 0.01], 2) the perchlorate discharge test was positive in 2 of 10 patients, and 3) the iodine-perchlorate discharge test, carried out in 7 of 8 patients with negative perchlorate discharge test, was positive in 6. Seven patients with the reversible type were given 25 mg iodine daily for 2-4 weeks; all became hypothyroid again. Two patients had a history of habitual ingestion of seaweed (25.4 and 43.1 mg iodine, respectively), but the remaining 10 patients ingested ordinary amounts of iodine (1-5 mg) daily. The patients with reversible hypothyroidism had focal lymphocytic thyroiditis changes in the thyroid biopsy specimen, whereas those with irreversible hypothyroidism had more severe destruction of the thyroid gland. These results indicate the existence of a reversible type of hypothyroidism sensitive to iodine restriction and characterized by relatively minor changes in lymphocytic thyroiditis histologically. Attention should be directed to this type of hypothyroidism, because thyroid function may revert to normal with iodine restriction alone.
103) BUCHANAN, W. WATSON, et al. “Iodine metabolism in Hashimoto’s thyroiditis.” The Journal of Clinical Endocrinology & Metabolism 21.7 (1961): 806-816.
A comprehensive study of iodine metabolism is reported in 40 cases of Hashimoto’s thyroiditis. The results showed a dissociation between the mean absolute (or stable) iodine uptake by the thyroid (2.0 ;μg. per hour, which was normal) and the serum level of protein-bound iodine (2.5 μg. per 100 ml., which was significantly decreased). These findings indicate that the thyroid gland traps a normal quantity of iodine, but lacks the capacity to utilize it efficiently to form thyroid hormone. This faulty utilization of iodine is apparently a form of acquired dyshormonogenesis. Evidence of its nature is provided by and by the presence of a butanol-insoluble iodinated protein in the plasma in many cases. The intrathyroidal exchangeable iodine was markedly reduced in almost all cases. This, and not the presence of the butanol insoluble iodinated protein, explains the frequent discrepancy between the high level of PBI131 and the low level of PBI in serum. Standard radioiodine tests (thyroidal uptake and plasma activity) yield misleading results unless the foregoing abnormalities in stable iodine metabolism are taken into account.
Our previous study of the iodide-perchlorate test in patients with Hashimoto’s thyroiditis led to us to assess the significance of the test in patients with Graves’ disease. Perchlorate tests were performed by a conventional method (ClO4 test) and a modified technique in which a dose of 250 or 500 μg 127I was added to the tracer 131I (I-ClO4 test).
104) SUZUKI, HOJI, and KEIMEI MASHIMO. “Significance of the iodide-perchlorate discharge test in patients with 131I-treated and untreated hyperthyroidism.” The Journal of Clinical Endocrinology & Metabolism 34.2 (1972): 332-338.
In the I-ClO4 test (500 μg 127I), a significant discharge was observed in the thyrotoxic patients, as well as in the patients rendered euthyroid by 131I treatment. With the smaller dose of 127I (250 μg), only 131I-treated patients showed significant iodide-perchlorate discharge. The magnitude of dischargeability (discharge percent) in the 131I-treated patients was greater than in thyrotoxic patients. ClO4 tests were negative in both the thyrotoxic and 131I-treated euthyroid patients. Chronic treatment with iodide induced hypothyroidism in 5 of 7 euthyroid patients who had been treated with 131I. The ClO4 test was negative in all patients except one, whereas the I-ClO4 test was positive in 6 of 7 patients. It would appear that the susceptibility of patients with Graves’ disease to acute and chronic iodide loads may be enhanced after 131I treatment, presumably by impairment of the thyroidal organic binding mechanism. The latter is more frequently detectable by the iodide-perchlorate test.
105) Andersen, B. Friis. “Iodide perchlorate discharge test in lithium-treated patients.” European Journal of Endocrinology 73.1 (1973): 35-42.
In patients with lithium-induced goitre the conventional perchlorate discharge test is negative. With a small dose of carrier iodide the perchlorate test is more sensitive. This iodide perchlorate discharge test (IPT) was made on twelve patients, all under lithium-treatment for over one year and on fifteen control subjects. In the first group there was a greater discharge in the test (Lithium prevents organification of Iodine) than in the latter group (Control). By increasing the lithium-doses or by stopping them, the discharge by the IPT alters correspondingly. Sensitivity to iodine is increased by lithium-treatment, perhaps because of a greater thyroid/serum ratio for iodide.
106) Morgans, M. E., and W. R. Trotter. “Defective organic binding of iodine by the thyroid in Hashimoto’s thyroiditis.” The Lancet 269.6968 (1957): 553-555.
107) Hilditch, T. E., et al. “Defects in intrathyroidal binding of iodine and the perchlorate discharge test.” European Journal of Endocrinology 100.2 (1982): 237-244.
The kinetics of [123I] iodide uptake were studied when organification of iodine by the thyroid gland was normal and when this binding function was diminished by drugs or disease. Each study was terminated by a sodium perchlorate discharge test (300–600 mg iv) at 60 min or, in some cases, 10–30 min. The results confirmed that binding takes place rapidly in the uninhibited gland with the binding rate constant being at least 0.150 min-1. Discharge from the uninhibited gland is less than 3.5% of the gland uptake when perchlorate is given 60 min after the radioiodide. Subjects with an intrinsic binding defect manifested discharges of 11% of greater of the 60 min uptake and the estimated binding rate constants ranged from 0.003–0.057 min-1. Thyrotoxic subjects receiving 5 mg carbimazole twice daily manifested discharges ranging from 5.4–64.2%, and in those receiving 20 mg twice daily the observed discharges were 67.6–94.6% of the 60 min uptake. The study shows that a correctly performed perchlorate discharge test will detect minimal inhibition of iodine binding. An important factor is the duration of the follow-up period after perchlorate is given. In some of the cases studied discharge was not complete until 60 min after the perchlorate.
108) Gray, H. W., et al. “Intravenous perchlorate test in the diagnosis of Hashimoto’s disease.” The Lancet 303.7853 (1974): 335-338.
In a study of patients with non-toxic goitre, the intravenous perchlorate-discharge test indicated the presence of defective organification of thyroidal iodide in nearly all patients with Hashimoto’s disease but in less than half those with simple goitre. Diagnostic discrimination was improved by setting limits of significant discharge at a minimum of 1% of the administered dose of iodine-131. Using this criterion, 80% of patients with Hashimoto’s disease but only 12% of patients with simple goitre had a positive test. In clinical practice, the test seemed most valuable in the differential diagnosis of non-toxic goitre in those euthyroid patients with low titres of serum-antithyroid-antibodies.
109) Bereda, G. “Hyperthyroidism: Definition, Causes, Pathophysiology and Management.” Journal of Biomedical and Biological Sciences 1.2 (2022): 1-11.
110) Van Doom, J., F. Roelfsema, and D. Van der Heide. “The effect of propylthiouracil and methimazole on the peripheral conversion of thyroxine to 3, 5, 3′-triiodothyronine in athyreotic thyroxine-maintained rats.” European Journal of Endocrinology 103.4 (1983): 509-5
111) BIANCO, ANTONIO C., et al. “Biochemistry, Cellular and Molecular Biology, and Physiological Roles of the Iodothyronine Selenodeiodinases.” Endocrine Reviews 23.1 (2002): 38-89.
112) Sharma, Anu, and Marius N. Stan. “Thyrotoxicosis: diagnosis and management.” Mayo Clinic Proceedings. Vol. 94. No. 6. Elsevier, 2019.
113) Pandey, Rahul, Sanjeev Kumar, and Narendra Kotwal. “Thyroid Storm: Clinical Manifestation, Pathophysiology, and Treatment.” Goiter-Causes and Treatment. IntechOpen, 2019.
114) Chopra, Inder J., et al. “Opposite effects of dexamethasone on serum concentrations of 3, 3′, 5′-triiodothyronine (reverse T3) and 3, 3′, 5′-triiodothyronine (T3).” The Journal of Clinical Endocrinology & Metabolism 41.5 (1975): 911-920.
115) Belshaw, Bruce E., and David V. Becker. “Necrosis of follicular cells and discharge of thyroidal iodine induced by administering iodide to iodine-deficient dogs.” The Journal of Clinical Endocrinology & Metabolism 36.3 (1973): 466-474.
116) Corvilain, Bernard, et al. “Stimulation by iodide of H2O2 generation in thyroid slices from several species.” American Journal of Physiology-Endocrinology and Metabolism 278.4 (2000): E692-E699.
117) Cohen, S. B., and A. P. Weetman. “The effect of iodide depletion and supplementation in the Buffalo strain rat.” Journal of endocrinological investigation 11 (1988): 625-627.
118) Rayman, Margaret P. “Multiple nutritional factors and thyroid disease, with particular reference to autoimmune thyroid disease.” Proceedings of the Nutrition Society 78.1 (2019): 34-44.
119) Fayadat, Laurence, et al. “Role of heme in intracellular trafficking of thyroperoxidase and involvement of H2O2 generated at the apical surface of thyroid cells in autocatalytic covalent heme binding.” Journal of Biological Chemistry 274.15 (1999): 10533-10538.
120) Okamura, Ken, et al. “Painless thyroiditis mimicking relapse of hyperthyroidism during or after potassium iodide or thionamide therapy for Graves’ disease resulting in remission.” Endocrine Journal (2022): EJ22-0207.
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Roti, E., et al. “Effects of chronic iodine administration on thyroid status in euthyroid subjects previously treated with antithyroid drugs for Graves’ hyperthyroidism.” The Journal of Clinical Endocrinology & Metabolism 76.4 (1993): 928-932.
Ten euthyroid women (mean age, 56 yr) who had hyperthyroid Graves’ disease successfully treated with methimazole 36.4 +/- 4.7 months earlier were evaluated before, during, and after the administration of 10 drops SSKI daily for 90 days.
Serum AbTPO increased slightly, but significantly, during SSKI administration in the 7 women with positive values at baseline (P < 0.05). TSH-RAb remained undetectable. After SSKI withdrawal, the 10 women were reevaluated 60 and 120 days later. Two women developed a blunted TSH response to TRH, but normal serum T4 and T3 concentrations, and 2 women developed overt hyperthyroidism, with undetectable basal and TRH-stimulated serum TSH and elevated serum T4 and T3 concentrations, requiring methimazole therapy. All values in the remaining 6 women were similar to those present before SSKI administration.
These results suggest that some euthyroid patients with a history of antithyroid drug therapy (Methimazole) for Graves’ disease may develop thyroid dysfunction during and after excess iodine administration. The development of subclinical hypothyroidism during SSKI administration was not clinically important, but the occurrence of overt hyperthyroidism after SSKI was discontinued did require antithyroid drug therapy. It is advisable, therefore, to avoid iodine-containing substances in euthyroid patients with a history of antithyroid drug therapy for Graves’ disease.
ROTI, E., et al. “Impaired intrathyroidal iodine organification and iodine-induced hypothyroidism in euthyroid women with a previous of postpartum thyroiditis.” The Journal of clinical endocrinology and metabolism 73.5 (1991): 958-963.
Postpartum thyroiditis (PPT) is common and occurs in 1.7 to 16.7% of pregnant women, depending upon the study population. Most of these women develop transient hypothyroidism and thyroid function usually returns to normal. We have studied 11 euthyroid women with a previous history of PPT to determine the incidence of subtle defects in thyroid function measured by iodide-perchlorate (I-ClO4) discharge tests and TRH tests and to determine whether these women would develop iodide-induced hypothyroidism.
Seven (64%) had positive I-ClO4 discharge tests and 5 (46%) had an abnormally high TSH response to TRH. Thyroid antimicrosomal and antithyroid peroxidase were positive in 8 women (73%) with a previous episode of PPT.
The administration of pharmacological amounts of iodide (10 drops of saturated solution of potassium iodide daily) for 90 days to these 11 women resulted in elevated basal and TRH stimulated serum TSH concentrations in 8 (72.7%) compared to TSH values during iodide administration to women who had never been pregnant. Antimicrosomal and antithyroid peroxidase concentrations did not change during iodide administration.
These findings strongly suggest that euthyroid women with a previous episode of PPT have permanent subtle defects in thyroid hormone synthesis and are inordinately prone to develop iodide-induced hypothyroidism, similar to findings previously reported in euthyroid subjects with Hashimoto’s thyroiditis, with a previous episode of painful subacute thyroiditis, or previously treated with radioactive iodine or surgery for Graves’ disease.
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Yu, Jiashu, et al. “Vitamin E ameliorates iodine-induced cytotoxicity in thyroid.” Journal of endocrinology 209.3 (2011): 299.
Rasheed, Rabab, and Sherif Arsanyos. “VITAMIN E AMELIORATES THE TOXIC EFFECT OF AMIODARONE ON THYROID GLAND IN RATS: A HISTOLOGICAL AND ULTRASTRUCTURAL STUDY.” Journal of Medical Histology 2.1 (2018): 57-68.
Sun, Rong, et al. “Protection of Vitamin C on Oxidative Damage Caused by Long-Term Excess Iodine Exposure in Wistar Rats.” Nutrients 14.24 (2022): 5245.
Abraham, Guy E., and David Brownstein. “Evidence that the administration of Vitamin C improves a defective cellular transport mechanism for iodine: A case report.” The Original Internist 12.3 (2005): 125-130.
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Ślebodziński, A. B. “Ovarian iodide uptake and triiodothyronine generation in follicular fluid: The enigma of the thyroid ovary interaction.” Domestic animal endocrinology 29.1 (2005): 97-103.
Monteleone, Patrizia, Pinuccia Faviana, and Paolo Giovanni Artini. “Thyroid peroxidase identified in human granulosa cells: another piece to the thyroid-ovary puzzle?.” Gynecological Endocrinology 33.7 (2017): 574-576.
Mutinati, M., et al. “Localization of thyrotropin receptor and thyroglobulin in the bovine corpus luteum.” Animal reproduction science 118.1 (2010): 1-6.
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additional references
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Toxic Effects of Iodine Excess
Leoni, Suzana G., et al. “Regulation of Thyroid Oxidative State by Thioredoxin Reductase Has a Crucial Role in Thyroid Responses to Iodide Excess.” Molecular Endocrinology 25.11 (2011): 1924.
Iodide excess induces reactive oxygen species production and cell toxicity. Here we analyze the effects of high I− doses in rat thyroids and in PCCl3 cells in the period comprising I− autoregulation (i.e. 0–72 h after I− administration), focusing on NIS expression, redox state, and the expression and activity of selenoproteins. Our results show that NIS mRNA inhibition by I− does not occur at the transcriptional level, because neither NIS promoter activity nor Pax8 expression or its binding to DNA was modulated. Because I− uptake was inhibited much earlier than NIS protein, and no effect was observed on its subcellular localization, we suggest that I− is inhibiting NIS in the plasma membrane. The increased reactive oxygen species production leads to an increase in thioredoxin reductase mRNA levels and enzyme activity, which reduces the oxidative stress. Inhibition of thioredoxin reductase at either gene expression or activity levels prevented NIS recovery, thus illustrating a new role played by this selenoprotein in the regulation of cell homeostasis and consequently in I− autoregulation.
Serrano-Nascimento, Caroline, Jamile Calil-Silveira, and Maria Tereza Nunes. “Posttranscriptional regulation of sodium-iodide symporter mRNA expression in the rat thyroid gland by acute iodide administration.” American Journal of Physiology-Cell Physiology 298.4 (2010): C893-C899.
Our data show for the first time that iodide regulates NIS mRNA expression at posttranscriptional level, providing a new mechanism by which iodide exerts its autoregulatory effect on thyroid.
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Khalifa, Maram, Hassaan B. Aftab, and Vitaly Kantorovich. ““Fueling the Fire”-Irish Sea-Moss Resulting in Jod-Basedow Phenomenon in a Patient With Grave’s Disease.” Journal of the Endocrine Society 5.Supplement_1 (2021): A906-A906.
Conclusion: Irish sea moss is a readily available herbal supplement with high, variable amounts of iodine. Despite little scientific evidence, it is often marketed to improve goiter amongst other health benefits. The recommended daily iodine intake per the FDA is 150 mcg. Higher amounts are expected to initially cause a short-lived suppression of thyroid function; the Wolff-Chaikoff effect, followed by “escape” and accelerated production of thyroid hormone in abnormal thyroid gland, known as Jod-Basedow phenomenon. In our case, the patient unknowingly worsened her underlying Grave’s disease due to the Jod-Basedow effect. Of note, apparently she had a longer than expected course of Wolff-Chaikoff effect preceding the thyrotoxic state due to sporadic irregular intake of sea moss. Discontinuing sea moss led to clinical and biochemical improvement of hyperthyroidism without requiring thionamide therapy.
Prakash, Ishita, Eric S. Nylen, and Sabyasachi Sen. “Lithium as an Alternative Option in Graves Thyrotoxicosis.” Case Reports in Endocrinology (2015). A 67-year-old woman was admitted with signs and symptoms of Graves thyrotoxicosis. Biochemistry results were as follows: TSH was undetectable; FT4 was >6.99 ng/dL (0.7–1.8); FT3 was 18 pg/mL (3–5); TSI was 658% (0–139). Thyroid uptake and scan showed diffusely increased tracer uptake in the thyroid gland. The patient was started on methimazole 40 mg BID, but her LFTs elevated precipitously with features of fulminant hepatitis. Methimazole was determined to be the cause and was stopped. After weighing pros and cons, lithium was initiated to treat her persistent thyrotoxicosis. Lithium 300 mg was given daily with a goal to maintain between 0.4 and 0.6. High dose Hydrocortisone and propranolol were also administered concomitantly. Free thyroid hormone levels decreased and the patient reached a biochemical and clinical euthyroid state in about 8 days. Though definitive RAI was planned, the patient has been maintained on lithium for more than a month to control her hyperthyroidism.
Lithium has been shown to increase the retention of radioactive iodine (RAI) in the thyroid of patients with Graves’ thyrotoxicosis [15], in turn, leading to improvement of the efficacy of this therapy. Lithium given before or concomitantly with RAI has been shown to provide more immediate control of hyperthyroidism, by decreasing the release of preformed thyroid hormone, without decreasing the uptake of the RAI [16]. This effect is reminiscent of the Wolff-Chaikoff effect, where increased iodine content inside the follicular cell blocks release of hormone. This effect, much like the effect of iodine, is transient. Older literature notes the similarity of lithium to iodine and recommends its use only for short-term, rapid suppression [17]. However, our case highlights the fact that lithium has several other mechanisms of action that contribute to long-term control of thyrotoxicosis.
Studies have shown that lithium doses of 600 mg−1000 mg daily (300 mg every 8 hours), as well as lithium blood levels of 0.6–1.2 mmol/L, are best to control thyrotoxicosis. To avoid toxicity, it is best if serum lithium levels are maintained < 1.0 mmol/L, around 0.5 mmol/L. [14].
Our case illustrates that a low therapeutic level of lithium even around 0.2 mmol/L is sufficient to suppress thyroid overactivity without causing side effects. Lithium at a low dose appears to be an effective antithyroid medication even for a few months.
Leung, Angela M., and Lewis E. Braverman. “Iodine-induced thyroid dysfunction.” Current opinion in endocrinology, diabetes, and obesity 19.5 (2012): 414.
Following the cloning of the sodium iodide symporter (NIS), which actively transports iodine into the thyroid, in 1996 by Dai et al. [11], the effect of excess iodine on thyroid function in the rat was revisited. In 1999, it was reported that in the normal rat thyroid, there was a marked decrease in NIS expression by 24 h following excess iodine administration [12]. This was accompanied by the disappearance of the inhibition of thyroid hormone synthesis and the resumption of normal thyroid function. Thus, it is likely that escape from the acute Wolff–Chaikoff effect is associated with decreases in NIS synthesis, resulting in a decrease in intrathyroidal iodine concentrations, and the resumption of normal thyroid hormone synthesis.
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