Clinical studies of exposure to perchlorate in the United States.

Perchlorate is a competitive inhibitor of the sodium/iodine symporter, decreasing the active transport of iodine into the thyroid. It was used as an antithyroid drug in the treatment of hyperthyroidism in the 1950s and 1960s but was discontinued because of the occasional occurrence of aplastic anemia. More recently, lower doses of perchlorate have been used successfully in the treatment of iodine-induced hyperthyroidism. There has been concern that naturally occurring perchlorate and industrial contamination of water supplies with perchlorate might pose a health hazard by inducing or aggravating underlying thyroid dysfunction. In a series of studies in normal volunteers, administration of perchlorate from 2 weeks to 6 months and in perchlorate production workers exposed intermittently to high levels of perchlorate for years, no abnormalities of circulating thyroid hormones, thyroid-stimulating hormone, thyroglobulin, or ultrasound evaluation of thyroid structure were observed even though the thyroid (123)I uptake was decreased in some studies. Further studies of the effects of perchlorate on thyroid function in normal volunteers will now be difficult to carry out due to the adverse publicity that perchlorate and the studies on its effect have received.