The Effect of Short-Term Low-Dose Perchlorate on Various Aspects of Thyroid Function

Perchlorate (ClO 4 ) salts are found in rocket fuel, fireworks, and fertilizer. Because of ground water contamination, ClO 4 has recently been detected in large public water supplies in several states in the 4-18 μg/L (parts per billion [ppb]) range. The potential adverse effect of chronic low level ClO 4 ingestion on thyroid function is of concern to the Environmental Protection Agency (EPA). The daily ingestion of ClO 4 at these levels would be magnitudes below the therapeutic effect level of hundreds of milligrams of ClO 4 used in treating hyperthyroidism. Studies were carried out in nine healthy male volunteers who had normal thyroid function and negative thyroid antibodies to determine whether the ingestion of 10 mg of ClO 4 daily (approximately 300 times the estimated maximum amount of ClO 4 consumed from the affected water supplies) would affect any aspect of thyroid function. They ingested 10 mg of ClO 4 dissolved in a liter of spring water during waking hours for 14 days. Baseline serum thyrotropin (TSH), free thyroxine index (FTI), total triiodothyronine (TT 3 ), 4-, 8-, and 24-hour thyroid 123 I uptakes (RAIU), serum and 24-hour urine ClO 4 , 24-hour urine iodine, complete blood count (CBC), and chemistry profile were determined. All blood and urine tests were repeated on days 7 and 14 of ClO 4 administration and thyroid RAIU on day 14 of ClO 4 administration. All tests were repeated 14 days after ClO 4 was discontinued. No effect of ClO 4 on serum thyroid hormone or TSH concentrations, urinary iodine excretion, CBC, or blood chemistry was observed. Urine and serum ClO 4 levels were appropriately elevated during the course of ClO 4 ingestion in all subjects, demonstrating compliance. By day 14 of ClO 4 administration, the 4-, 8-, and 24-hour thyroid RAIU values decreased in all nine subjects by a mean value of 38% from baseline and rebounded above baseline values by 25% at 14 days after ClO 4 withdrawal (p < 0.01 analysis of variance (ANOVA) and Tukey). It is well known that the major effect of ClO 4 on the thyroid is a decrease in the thyroid iodide trap by competitive inhibition of the sodium iodide symporter (NIS). The present study demonstrates the sensitivity of the thyroid iodide trap to ClO 4 because a low dose of 10 mg daily significantly decreased the thyroid RAIU without affecting circulating thyroid hormone or TSH concentrations. It is possible, however, that the daily consumption of low levels of ClO 4 in drinking water over a prolon