lhe Effect of Iodide Administration on the Serum TSH and Ts Responses to TRH * Maximum A Serum Ts 120 A Ts

tropin (TSH) responsiveness to thyrotropin-releasing hormone (TRH) is enhanced by small decreases in serum thyroxine (To) and triiodothyronine (Ti), 12 euthyroid volunteers were given 190 mg iodide po daily for 10 days to inhibit T4 and Tt release from the thyroid. Basal serum To, T3, and TSH concentrations and the serum T4 and TSH responses to 400 iug TRH i.v. were assessed before and at the end of iodide administration. Iodide induced small but highly significant decreases in basal serum To (8.0±1.6 vs. 6.6±1.7 ug/lOO ml; mean + SD) and T3 (128±15 vs. 110±22 ng/100 ml) and increases in basal serum TSH (1.3±0.9 vs. 2.1±1.0 gU/ ml). During iodide administration, the TSH response to TRH was significantly increased at each of seven time points up to 120 min. The maximum increment in serum TSH after TRH increased from a control mean of 8.8± 4.1 to a mean of 13.0+2.8 ALU/ml during iodide administration. As evidence of the inhibitory effect of iodide on hormonal release, the increment in serum Ts at 120 min after TRH was significantly lessened during iodide administration (61±42 vs. 33±24 ng/100 ml). These findings demonstrate that small acute decreases in serum T4 and Ts concentrations, resulting in values well within the normal range, are associated both with slight increases in basal TSH concentrations and pronounced increases in the TSH response to TRH. These results demonstrate that a marked sensitivity of TSH secreThis work was presented in part at the Annual Meeting of the American Federation for Clinical Research, Atlantic City, N. J., May 1974. Received for publication, 10 April 1974 and in revised form 6 June 1974. tion and responsiveness to TRH is applicable to decreasing, as well as increasing, concentrations of thyroid hormones.