Effect of insulin on thyroid hormone blood levels in normal and thyroidectomized dogs.

Administration of insulin brings about an increase of thyroid hormone blood concentration. This study investigated the possibilities that the elevation is due to increased thyroid secretion or to liberation of hormone from hepatic stores. Monocomponent insulin was administered to intact dogs by jugular perfusion (25 mU/kg/min. for 15 min.) and to thyroidectomized dogs either as single dose i.m. (1 U/kg) or by jugular perfusion (as above). All animals showed at least halving of blood glucose concentration. In normal dogs, circulating labeled thyroid hormone increased with a 2-fold T3 rise. In thyroidectomized dogs, blood T3 as well as T4 remained persistently low, while plasma 125I-T4 disappearance slope was unchanged. When intact animals were held in a glycemic steady state by infusing insulin alone at low dose (0.5 mU/kg/min. for 15 min.) or insulin at the experimental dose (25 mU/kg/min. for 15 min) with simultaneous glucose infusion, blood T3, T4 and labeled PBI showed no changes. These data suggest that the increase of circulating thyroid hormones under insulin administration is due to a stimulation of thyroid secretion. The effect of insulin appears to result from induced hypoglycemia.