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Too Much of a Good Thing - Iodine: 1 mg/day + Can be Bad for You, But the Dangers are as Individual as the Benefits

Seafood is an excellent source of iodine, but many people get most of their iodine from iodized table salt.

I assume you would have expected me to forget that I promised to do my very best to turn the "too much of a good thing" article on selenium I wrote a week ago into a series, right? Well, you obviously don't know me well enough, then, because here's installment #2 of this new series. One that is closely related to #1, but was significantly more difficult to write.

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 important role of iodine in brain and general physical development and the ~30% increase in iodine requirements in pregnant women (Delange. 2000; Zimmermann. 2004 & 2011),
• 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)

are concerned. What I would like to mention, at least, is the often-cited role of iodine in breast cancer, because the purported protective effect of iodine from supplements or food (Cann. 2000; Funahashi. 2001) is one of the reasons many women consume iodine supplements in dosages that could be more of a threat than a blessing for their health. That's bad news in view of the fact that the "evidence" of breast cancer protective effects is still in the hypothetical state and "rigorous retrospective and prospective studies are needed to confirm this hypothesis" (Cann. 2000).
Things we can say about iodine that are above all relevant for both, men and women, is that it is essential for normal thyroid function. Just as it is the case for selenium (discussed in the last installment of this series), though, the effects of supplemental iodine on thyroid function are far from being conclusive. While a lack of iodine will force the thyroid to increase it's efficacy by - eventually - developing goiterous growth, the dietary and even more so the supplemental ingestion of copious amounts of iodine (>1mg) do something most people won't expect: They reduce the function of the thyroid glands.

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).

It is thus not surprising that populations with low and high iodine intake tend to develop opposite thyroid disorders with

• 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). 

This does not mean, that "half-deficient" intakes were best, even though there's evidence that a mild = non-goitrogenic iodine deficiency partly protects against autoimmune thyroid disease (Laurberg. 1998). In that US scientists warn that it is not sure, in how many cases the iodine induced reduction in thyroid function "may result from the adverse effect of excess iodine on thyroid function in persons with underlying thyroid disease such as Hashimoto’s thyroiditis, history of subacute thyroiditis, or silent postpartum lymphocytic thyroiditis" (Hollowell. 2002).
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

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)

"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). 

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.

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).

Overall, it should be noted, though that the human body is pretty good at managing high intakes of iodine. While iodine-induced hypothyroidism was also reported in patients with no apparent thyroid disease the number of scientifically verified reports like the mild reversible hypothyroidism that was observed in elderly patients consuming iodinated glycerol by Drinka et al. (1998) or the two Japanese patients with anorexia nervosa developed iodine-induced hypothyroidism after they had consumped of large quantities of OshaberiKombu, a Japanese food that is extensively consumed on lowe energy diets and contains approximately 13.4 mg of iodine per 100g (Matsubayashi. 1998).

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).
References:

• Cann, Stephen A., Johannes P. van Netten, and Christiaan van Netten. "Hypothesis: iodine, selenium and the development of breast cancer." Cancer Causes & Control 11.2 (2000): 121-127.
• Caldwell, Kathleen L., et al. "Iodine status of the US population, National Health and Nutrition Examination Survey, 2005–2006 and 2007–2008." Thyroid 21.4 (2011): 419-427.
• Delange, François. "The disorders induced by iodine deficiency." Thyroid 4.1 (1994): 107-128.
• Delange, Francois. "The role of iodine in brain development." Proceedings of the nutrition society 59.01 (2000): 75-79.
• Denef, Jean-François, Marie-Christine Many, and M. F. van Den Hove. "Iodine-induced thyroid inhibition and cell necrosis: two consequences of the same free-radical mediated mechanism?." Molecular and cellular endocrinology 121.1 (1996): 101-103.
• Drinka, Paul J., and Wolfram E. Nolten. "Effects of iodinated glycerol on thyroid function studies in elderly nursing home residents." Journal of the American Geriatrics Society 36.10 (1988): 911-913.
• Fuge, R. "Iodine in the environment: its distribution and relationship to human health." Trace substances in environmental health-XXI. 1987.
• Funahashi, Hiroomi, et al. "Seaweed prevents breast cancer?." Cancer Science 92.5 (2001): 483-487.
• Hetzel, Basil S. "Iodine and neuropsychological development." The Journal of nutrition 130.2 (2000): 493S-495S.
• Hollowell, Joseph G., et al. "Serum TSH, T4, and thyroid antibodies in the United States population (1988 to 1994): National Health and Nutrition Examination Survey (NHANES III)." The Journal of Clinical Endocrinology & Metabolism 87.2 (2002): 489-499.
• Laurberg, Peter, et al. "Iodine intake and the pattern of thyroid disorders: a comparative epidemiological study of thyroid abnormalities in the elderly in Iceland and in Jutland, Denmark." The Journal of Clinical Endocrinology & Metabolism 83.3 (1998): 765-769.
• Many, Marie-Christine, et al. "In vitro study of acute toxic effects of high iodide doses in human thyroid follicles." Endocrinology 131.2 (1992): 621-630.
• Matsubayashi, Sunao, et al. "Iodine-induced hypothyroidism as a result of excessive intake of confectionery made with tangle weed, Kombu, used as a low calorie food during a bulimic period in a patient with anorexia nervosa." Eating and Weight Disorders-Studies on Anorexia, Bulimia and Obesity 3.1 (1998): 50-52.
• 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.
• Roti, E., and A. G. Vagenakis. "Effect of excess iodide: clinical aspects." Werner and Ingbar’s The Thyroid–Braverman LE and Utiger RD. Ed, Lippincott (2000): 316-329.
• 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.
• Zanzonico, Pat B., and David V. Becker. "Effects of time of administration and dietary iodine levels on potassium iodide (KI) blockade of thyroid irradiation by 131I from radioactive fallout." Health Physics 78.6 (2000): 660-667.
• Zimmermann, M., and F. Delange. "Iodine supplementation of pregnant women in Europe: a review and recommendations." European Journal of Clinical Nutrition 58.7 (2004): 979-984.
• Zimmermann, Michael B. "The role of iodine in human growth and development." Seminars in cell & developmental biology. Vol. 22. No. 6. Academic Press, 2011.