Objective: To review the epidemiological literature relevant to evaluating the risk of adverse effects on thyroid function associated with environmental perchlorate exposure.

Methods: All studies investigating possible adverse effects of environmental perchlorate exposure on thyroid function in adults or in pregnant women and their newborns were critically evaluated.

Results: There is no credible or consistent evidence from any of the numerous studies using a variety of designs that environmental exposure to perchlorate has any adverse effect on thyroid function, whether measured as changes in thyroid hormone levels or, among newborns, as the diagnosis of primary congenital hypothyroidism. The absence of adverse effects of environmental perchlorate on the thyroid likely reflects the very low perchlorate exposure levels worldwide, compared with much higher levels of exposure to nitrate and thiocyanate, goitrogens that inhibit thyroid function by the identical mechanism of action proposed for perchlorate. Adjusted for potency, perchlorate accounts for <1% of the inhibition of iodide uptake in the thyroid resulting from environmental exposure to nitrate and thiocyanate. Occupational studies of workers with substantially higher perchlorate exposure than the general population indicate that even extended exposure to high perchlorate levels does not adversely affect thyroid function.

Conclusion: There is no epidemiologic evidence that environmental or occupational exposure to perchlorate adversely affects thyroid function in the United States. Even if all perchlorate could be removed from the environment, >99% of the inhibition of iodide uptake in the thyroid resulting from exposure to environmental goitrogens would remain.