The Relative Contribution of Thyroxine and Triiodothyronine to the Hormone Secretion from the Perfused Canine Thyroid During Various Degrees of Stimulation

Abstract

The effect of constant infusions of bovine TSH [thyrotropin] in different concentrations on the pattern of T4 [thyroxine] and T3 [triiodothyronine] secretion was examined in perfused canine thyroids isolated in situ. Each of the thyroid lobes was isolated and perfused in each animal. It was possible to examine the effect of 2 different concentrations of TSH in the same animal. The latency period was shorter and the increase in hormone release was steeper when a high concentration of TSH (100 .mu.U[microunits]/ml) was infused than after a low concentration of TSH (2 .mu.U/ml). The increase in T4 and T3 release started simultaneously; at all TSH concentrations employed a phase of relatively high secretion of T3 was induced. Despite continued infusion of TSH, the T4/T3 ratio in the effluent always returned to near prestimulation values at the end of the experiment. There was no support for a sustained enhancement of the preferential secretion of T3 from the thyroid during prolonged stimulation of the thyroid.