Hypothyroidism can be provoked by small amounts of supplemental iodine

Summary: Care must be taken when considering iodine supplementation because it can provoke latent thyroid autoimmunity resulting in hypothyroidism.A noteworthy study just published in The American Journal of Clinical Nutrition adds more evidence that iodine supplementation, even in small amounts, can produce hypothyroidism. The authors state:

"The beneficial health effects associated with Universal Salt Iodization are well known. Yet, little is known about the possible adverse health effects in people with high iodine intake and the safe daily intake upper limit in the Chinese population...The objective of this study was to explore the safe upper level of total daily iodine intake among adults in China."

They examined 256 adults with apparently normal thyroid function in a 4 week double-blind, placebo-controlled, randomized controlled trial. The subjects were randomly assigned to 12 different levels of iodine supplementation ranging from 0 to 2000 micrograms per day (2000 μg = 2 milligrams). Iodine from both supplements and diet was taken into consideration. They were then evaluated for thyroid function, thyroid size, and urinary iodine. The outcome was striking for what would seem to be a modest amount:

"The mean iodine intake from the diets and salt intake of the participants were 105 ± 25 and 258 ± 101 μg/d, respectively. In comparison with the placebo group, all iodide-supplemented groups responded with significant increases in median urinary iodine concentrations and in thyroid-stimulating hormone concentration. Thyroid volume decreased after 4 wk in the high-iodine intervention groups (1500–2000 μg). Subclinical hypothyroidism appeared in the groups that received 400 μg I (5%) and 500–2000 μg I (15–47%)."

This is striking in that even 400 micrograms, only 0.4 milligrams, provoked subclinical hypothyroidism in a significant percentage of patients. This is why I published an earlier post regarding the need for care in the use of iodine for radiation protection, to say nothing of the inappropriate supplementation of large amounts of iodine without due care. In this study the highest intervention group which was still only 2 mg per day had noticeable thyroid shrinkage. The authors conclude:

"This study showed that subclinical hypothyroidism appeared in the participants who took the 400-μg I supplement, which provided a total iodine intake of ∼800 μg/d. Thus, we caution against a total daily iodine intake that exceeds 800 μg/d [0.8 milligrams] in China and recommend further research to determine a safe daily upper limit."