|Seafood is an excellent source of iodine, but many people get most of their iodine from iodized table salt.|
Why's that? Well, if you took a look at the literature discussing the effects of iodine supplementation on thyroid function, you'd know why it was ... well, "complicated" and not exactly unambigous to say the least.
In view of the fact that it would go beyond the scope of any single article to discuss the detailed effects of iodine and iodine supplements in the various thyroid disease states, I will focus solely on the beneficial and detrimental effects of iodine in more or less healthy individuals. Furthermore, I will not go into details as far as...
Table 1: The spectrum of iodine deficiency disorders - click to see all (Delange. 1994)
- the role of iodine in psychiatric disease (Hetzel. 2000), and
- the influence of (inorganic) iodine in the environment on human / general mammalian health (Fuge. 1987)
|In the subjects who received more than 1mg of supplemental iodine thyroid size & function was reduced in less than 4 weeks (Sang. 2012).|
|Figure 1: On a population-based study, high(er) iodine intakes are associated with higher TSH levels and thus reduced thyroid function, while high intakes are associated with increased TSH and reduced thyroid fct. (Laurberg. 1998).|
- goiter and thyroid hyperfunction being more prevalent among populations with low(er) iodine intakes, and
- impaired thyroid function (=subclinical and clinical hypothyroidism) being more prevalent among populations with relatively high iodine intakes (Lauerberg. 1998).
The Wolff-Chaikoff effect, a supposedly transient reduction in thyroid function occurs in response to increases in iodine intake and is mediated by a decrease intrathyroidal inorganic iodine uptake into the thyroid (it shuts off) by down regulation of the sodium iodine symporter (NIS). The effect is also supposed to save your thyroid from the assault of radioactive iodine particles in the case of a fallout. Therefore the same 1mg+ iodine tablets people consider a "health supplement" these days were considered a fallout emergency medication during Cold War (Zanzonico. 2000).It is thus all the more important to take the little experimental evidence there is into consideration. In that, the often-seen drops in thyroid function in response to the ingestion of large quantities of iodine are often ascribed to the so-called Wolff-Chaikoff which is supposed to reduce the thyroid function for no longer than a 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 (Markou. 2001). Now, the problem is: The phenomenon is not always as short-lived as it is supposed to be
Even if you believe you know you don't suffer from any of the aforementioned thyroid problems taking 10mg or more of iodine may be likened to Russian Thyroid Roulette, because it always entails the risk of iodide-induced hypothyroidism. Usually the hypothyroidism is transient and regresses within 2-3 weeks after iodide withdrawal, but as Markou et al. highlight there's a small subgroup of patients who develop transient iodine-induced hypothyroidism that must be followed long term thereafter because many will develop permanent primary hypothyroidism.
"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 [and may require temporary or permanent treatment with thyroid medications]" (Markou. 2001).
Table 2: Taking high amounts of iodine can significantly reduce thyroid function or induce goiter. Specifically in the subgroups of the population mentioed above (Roti. 2000)
|Figure 2: People with a previous history of painful subacute thyroiditis (SAT) are highly susceptible to the hypothyroid effects of 300mg/day iodine compared to people who had surgery for benign thyroid nodules (Roti. 1990).|
All in all the aforementioned examples are yet the exception to the rule. A rule which is evidenced in studies like Markou et al. (2000), in which the authors administered 80 mg potassium iodide (which is also the form of "iodine" in Lugol's solution) to 30 normal children 8 to 14 years old for 3 months and found that none developed overt or subclinical hypothyroidism. That does not mean that the iodine was without effect, but what the scientists observed was the expected transient elevation in serum TSH values (5–8 mU/L) as a result of the Wolff-Chaikoff effect. An effect that abated without further medical intervention on part of the researchers after 3-4 weeks when the TSH returned to normal levels despite the continuation of iodide treatment (Markou. 2000).
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- Funahashi, Hiroomi, et al. "Seaweed prevents breast cancer?." Cancer Science 92.5 (2001): 483-487.
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- Markou, K., et al. "Effects of chronic iodide administration in thyroid function in children and adolescents with a history of transient congenital hypothyroidism." 27th Annual Meeting, Greek Endocrine Association, Nicosia Cyprus. Vol. 82. 2000.
- Roti, Elio, et al. "Iodine-Induced Hypothyroidism in Euthyroid Subjects with a Previous Episode of Subacute Thyroiditis*." The Journal of Clinical Endocrinology & Metabolism 70.6 (1990): 1581-1585.
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- Sang, Zhongna, et al. "Exploration of the safe upper level of iodine intake in euthyroid Chinese adults: a randomized double-blind trial." The American journal of clinical nutrition 95.2 (2012): 367-373.
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