The relative contribution of thyroxine and triiodothyronine to the hormone secretion from the perfused canine thyroid during various degrees of stimulation

The effect of constant infusions of bovine TSH in different concentrations on the pattern of T4 and T3 secretion was examined in perfused canine thyroids isolated in situ. Each of the thyroid lobes was isolated and perfused in each animal. It was thus possible to examine the effect of two different concentrations of TSH in the same animal. The latency period was found to be shorter and the increase in hormone release was steeper when a high concentration of TSH (100 microU/ml) was infused than after a low concentration of TSH (2 microU/ml). The increase in T4 and T3 release started simultaneously; however, 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. Accordingly, there was no support for a sustained enhancement of the preferential secretion of T3 from the thyroid during prolonged stimulation of the thyroid.