Dietary iodine intake in the United States is greater than that considered necessary for the maintenance of normal thyroid function. The administration of pharmacologic quantities of iodine (10 to 1,000 mg daily) to euthyroid subjects results in small decreases in the serum T4 and T3 concentrations and a compensatory increase in the basal and TRH-stimulated serum TSH concentrations. Studies were carried out to determine whether a far smaller increase in iodine intake would also affect thyroid function. Normal volunteers received 1,500, 500, or 250 micrograms supplemental iodine daily for 14 days. Following the administration of 1500 micrograms iodine daily, there were small but significant decreases in the serum T4 and T3 concentrations and a small compensatory increase in the serum TSH concentration and the serum TSH response to TRH. In contrast, no changes in pituitary-thyroid function occurred during the administration of 500 or 250 micrograms iodine daily. These findings indicate that a small increase in dietary iodine can induce subtle changes (all values remaining within the normal range) in pituitary-thyroid function, probably by inhibiting thyroid hormone release. The smaller iodine supplements of 500 and 250 micrograms daily, quantities that may easily be achieved under normal conditions, did not, however, affect thyroid function.