Effects of six months of daily low-dose perchlorate exposure on thyroid function in healthy volunteers

Context: Perchlorate has been detected in U. S. drinking water supplies at levels ranging from 4 to 200 mu g/liter as well as in agricultural products. Perchlorate is known to be a competitive inhibitor of iodine uptake by the thyroid through the sodium-iodide symporter. Objective: The objective of the study was to determine whether prolonged exposure (6 months) to low levels of perchlorate would perturb thyroid function. Design: This was a prospective, double-blinded, randomized trial. Participants: The study population consisted of 13 healthy volunteers. Intervention: Interventions included placebo vs. 0.5 mg or 3.0 mg potassium perchlorate daily. Main Outcome Measures: Serum thyroid function tests, 24-h radioactive iodine uptake, serum thyroglobulin (Tg), urinary iodine and perchlorate, and serum perchlorate were measured. Results: Mean urinary perchlorate value during ingestion of 0.5 mg perchlorate daily was 332.7 +/- 66.1 mu g per 24 h or 248.5 +/- 64.5 mu g/g creatinine and mean values for the four subjects who received 3 mg perchlorate daily were 2079.5 +/- 430.0 mu g per 24 h or 1941.7 +/- 138.5 mu g/g creatinine. There was no significant change in the thyroid I-123 uptakes during perchlorate administration. There were no significant changes in serum T-3, free T-4 index, TSH, or Tg concentrations during the exposure period, compared to baseline or postexposure values. Urine iodine values for the 3-mg perchlorate group were higher, but not significantly so, at baseline than during perchlorate exposure. Conclusions: We observed that a 6-month exposure to perchlorate at doses up to 3 mg/d had no effect on thyroid function, including inhibition of thyroid iodide uptake as well as serum levels of thyroid hormones, TSH, and Tg.