Perchlorate and thyroid function: Is iodine supplementation protection enough?

Perchlorate, an oxidizing agent used primarily by the military as a component of rocket fuel, is a widespread food and drinking water contaminant. Recent evidence suggests that low-level perchlorate exposures common in the U.S. may decrease thyroid hormone function in women with moderately low iodine nutrition. This has led to the suggestion that iodine supplementation may be an effective means to reduce the potential harm of perchlorate as well as that of other endocrine disruptors that similarly inhibit iodine uptake by the thyroid . However, observational evidence suggests that high iodine intake is also associated with thyroid disorders. This study uses data from the 2001-2 National Health and Nutrition Survey (NHANES) to examine two relationships: that between 1) high levels of iodine and thyroid hormone, and 2) perchlorate and thyroid hormone for women with more than adequate iodine nutrition. Women in the highest urine iodine category (> 600 µg/L) had median thyroid hormone (T4) levels that were lower and median thyroid stimulating hormone (TSH) levels that were higher than women with iodine in the middle and lower ranges, although these differences were not statistically significant. A statistically-significant association between T4 and urinary perchlorate was found in women with levels of urinary iodine in the upper 95th percentile of U.S women. This result suggests that high as well as low iodine intake interacts with perchlorate to disrupt thyroid function. Because blanket iodine supplementation may result in more women with excess iodine intake, it may not protect the public against the effects of perchlorate exposure.