TIANJIN, CHINA. Many alternative health practitioners and web sites promote supplementation with iodine in amounts vastly exceeding the recommended daily allowance (RDA) of 150 microgram/day and the official North American upper safe limit of 1100 microgram/day. Some iodine supplementation proponents recommend daily intakes of 20,000 to 50,000 microgram/day. Among purported benefits, improved thyroid function and the prevention or cure of breast cancer, hemorrhoids and migraine headaches. High-dose iodine supplementation is also claimed to strengthen the immune system, prevent the growth of harmful bacteria in the gut, and flushes out chemical toxins from the body (including fluoride, lead and mercury). An obvious question, is it safe to supplement with 20 to 50 times the official safe limit?

A recent study carried out in Denmark concludes that the incidence of hypothyroidism (low thyroid gland functioning) has increased since mandatory iodization of household salt was implemented in 2000 and two studies, one Danish and one Chinese, warn of a possible connection between an increase in thyroid cancer and increased iodine intake.[1-3]

Now a group of researchers at the School of Public Health in Tianjin report that high intakes of iodine are associated with the development of subclinical hypothyroidism. Their 4-week double-blind, placebo-controlled clinical trial included 140 healthy young men and 116 healthy young women with normal thyroid function with an average age of 22 years. The study participants were randomly assigned to one of 12 groups according to the amount of supplemental iodine prescribed (covering the range from 0 microgram/day to 2000 microgram/day of elemental iodine in the form of potassium iodide). In addition to iodine from supplements, the participants also ingested an average 110 microgram/day from the diet and 258 microgram/day from iodized salt.

At the 2-week mark urinary iodine excretion had nearly doubled in the group receiving 400 microgram/day of supplemental iodine, while it had increased by a factor of 6 in the group supplementing with 2000 microgram/day. The serum concentration of free triiodothyronine (FT3) did not change significantly over the 4-week trial period irrespective of iodine intake. However, at 4 weeks free thyroxine (FT4) concentrations were significantly lower in the 500, 750, 1250 and 1500 microgram/day supplementation groups when compared to the placebo group and the 200 and 400 microgram/day groups. The most noticeable change in serum thyroid hormones involved thyroid-stimulating hormone (TSH). At the end of the 4-week study period, TSH concentrations were significantly higher (4.4 mIU/L) in the 1250, 1500, 1750 and 2000 microgram/day supplementation groups than in the placebo group (2.4 mIU/L). Although the "official" normal range of TSH is 0.2 to 5.5 mIU/L, there is growing evidence that a range of 0.5 to 3.5 mIU/L would be more realistic, with values above 3.5 mIU/L indicating hypothyroidism.

The Chinese researchers conclude that 5% of study participants supplementing with 400 microgram/day and 47% of those supplementing with 2000 microgram/day developed subclinical hypothyroidism during the 4-week trial. They suggest that the total daily intake of iodine (from diet, salt and supplements) should not exceed 800 microgram/day in healthy individuals.
Sang, Z, et al. Exploration of the safe upper level of iodine intake in euthyroid Chinese adults: a randomized double-blind trial. American Journal of Clinical Nutrition, Vol. 95, 2012, pp. 367-73

Editor's comment: The finding that iodine supplementation may be associated with the development of hypothyroidism is unexpected and somewhat counterintuitive since a high iodine intake is generally associated with improved thyroid health. Several afibbers have found iodine supplementation beneficial. Nevertheless, it would seem prudent to keep an eye on TSH levels if supplementing with iodine and perhaps to rethink high-dose iodine supplementation altogether if already diagnosed with clinical hypothyroidism.
[1] Laurberg, P, et al. Iodine fortification of salt and thyroid disease in Denmark. Ugeskr Laeger, Vol. 173, No. 50, December 12, 2011, pp. 3264-70 [article in Danish – English abstract only]
[2] Blomberg, M, et al. Thyroid cancer in Denmark 1943-2008, before and after iodine supplementation. International Journal of Cancer, February 15, 2012 [Epub ahead of print]
[3] Wang, Y and Wang, W. Increasing incidence of thyroid cancer in Shanghai, China, 1983-2007. Asia Pacific Journal of Public Health, February 16, 2012 [Epub ahead of print]