Control of thyroid hormone secretion in normal subjects receiving iodides

The administration of exogenous iodides (saturated solution of potassium iodide, SSKI) to normal male volunteers resulted in a significant decrease in the serum concentration of thyroxine (T(4)) and triiodothyronine (T(3)) and a significant increase in serum concentration of thyrotropin (TSH). During the control period (phase I), serum concentrations of T(4) averaged 6.9+/-1.8 mug/100 ml (mean +/-SD), T(3) 106+/-15 ng/100 ml, and TSH 3.7+/-1.3 muU/ml. During the administration of 1 drop of SSKI twice daily for 11 days (phase II), there was a small but significant decrease in the serum concentration of T(4) and T(3) (5.8+/-1.6 mug/100 ml and 91+/-19 ng/100 ml, respectively) and a small but significant increase in the serum concentration of TSH (6.0+/-3.5 muU/ml). During the administration of 5 drops of SSKI twice daily (phase III) over the following 12-19 days, these changes persisted, except for a small increase in the serum concentration of T(3) (97+/-20 ng/100 ml), which was statistically significant when compared to values obtained during phase II. Values returned to control levels 14 days after withdrawal of SSKI. Almost all these observed changes took place within the limits of the normal range. It is postulated that, in euthyroid individuals, iodides specifically inhibit release of T(4) and probably of T(3). The resulting slight decrease in values for serum T(4) and T(3) elicits a small increase in TSH secretion which, it is postulated, antagonizes the inhibition of hormone release induced by iodides. As a result, a new equilibrium is reached which maintains the euthyroid state.