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.

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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).

In the subjects who received more than 1mg of supplemental iodine thyroid size & function was reduced in less than 4 weeks (Sang. 2012).

So what's the safe upper intake limit? I would be hesitant to give a one-size-fits it all limit, because in some people even little more than the RDA may be a problem. According to a 2012 study from the Tianjin Medical University subclinical hypothyroidism in the form of decreased thyroid volumes and TSH levels can occur with as little of 400mcg/day, i.e. 0.4mg/day, of supplemental iodine and a total iodine intake of 800 mcg/day (Sang. 2012). In view of the fact that it took up 500-2000mcg/day to induce a similar hypothyroid state in almost 50% of the subjects, I can only repeat that there's no "one-size-fits-it-all cut-off" for iodine.

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).

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

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).

So what's the verdict? While it is possible to consume "too much of a good thing" for iodine, it is still not clear (a) how much too much is and (b) for whom this really is a problem. Assuming that you already have thyroid issues or you realize that something is wrong with your TSH, T3 or T4 values after starting to supplement with iodine, stop taking it - the evidence that there are real benefits of consuming iodine in the milligram-to-gram range on a daily basis is quasi non-existent, anyways and evidence of a linear or predictable positive relationship between the amount of iodine you consume and your metabolic rate is as non-existent as proven beneficial effects on the gut microbiome, iodine's ability to "flush out toxins" or the "potent anti-oxidant effects" you can read about on the Internet (in fact, high doses are pro-oxidant and may even have necrotic effects, meaning that the will kill cells - good or bad | Many. 1992; Denef. 1996).

Too Much of a Good Thing - Selenium: Little More Than in Many 'Multis' Can Trigger Subclinical Hypothyroidism, Less Cuts 13% Body Fat | more

That being said, the supply of iodine in the US diet is generally sufficient (Caldwell. 2011), only those groups who are unfortunately also most likely to get too little of other good stuff, i.e. those parts of the population who eat the most messed up diets appear at a small risk of not getting enough iodine on a daily basis. If you have kids or are planning to have kids, though, you may still want to screen your diet for adequate intakes which would be 90 mcg/day for children, 150 mcg/day for adults and 220 mcg/day (but not more than 300-400mcg) for pregnant and 290 mcg/day for lactating/breastfeeding women | Comment on Facebook!

References:

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