Tablets containing tetraglycine hydroperiodide are used to purify small quantities of water for drinking. Because short-term administration of stable iodide can alter thyroid function in normal adults, we evaluated the effects of these tablets on thyroid function during a military field training exercise. Fourteen normal volunteers participated in the study. After an adjustment period of 2 weeks to the field environment, half of the subjects received four dissolved water-purification tablets (32 mg free iodine) daily for 7 consecutive days while the other half served as controls. At the end of the study the treatment group showed reductions in the mean serum levels of thyroxine (14%) and total triiodothyronine (15%) and significant increments in basal thyrotropin (TSH) (122%) and the TSH response to thyrotropin-releasing hormone (49%) compared to baseline values. The basal TSH level rose above the normal range in two subjects. No significant changes in any parameter of thyroid function were observed in the control subjects. One week of daily exposure to the iodine load from four tetraglycine hydroperiodide water-purification tablets causes mild impairment of thyroid function in humans.