Evidence That Thyroxine Inhibits either Basal or TRH-Induced TSH Secretion Only after Conversion to Triiodothyronine 1

Abstract

When TRH was administered every 15 min for 2 hr in euthyroid rats, equivalent modestly supraphysiologic doses of either T₄ or T₃ suppressed TRH-induced TSH secretion after 45 min. Pretreatment with iopanoic acid blocked the ability of T₄ but not of T₃ to suppress TRH-induced TSH secretion 2 hr after administration of the respective thyroid hormone. Pretreatment with iopanoic acid also blocked the ability of T₄, but not of T₃, to depress the elevated basal plasma TSH concentration of hypothyroid rats within 2 hr. Propylthiouracil did not significantly inhibit the ability of T₄ to depress TRH-induced TSH secretion and only slightly depressed the ability of T₄ to reduce the elevated plasma TSH of hypothyroid rats. Our data support the concept that although equivalent physiologic doses of T₄ or T₃ inhibit basal or TRH-induced TSH secretion equally rapidly, TSH inhibition produced by T₄ is probably dependent on its rapid conversion to T₃, either within the pituitary or peripherally. T₃ thus seems to be exerting almost all the negative feedback effects on TSH secretion under the conditions of our experiments.