Dose-response studies on the inhibitory effect of thyroid hormones on insulin secretion in the rat

Dose-response studies have been performed to investigate the effect of thyroid hormones on insulin secretion from the rat pancreas with special reference to the time course of the hormone effect, doses of triiodothyronine (T3) and thyroxine (T4), and glucose concentration in the perfusion medium. The prominent effect of thyroid hormones was the inhibition of the late phase of glucose-induced insulin secretion. As the late phase comprises at least 98%-99% of insulin released from the pancreas during a 60-min stimulation period with glucose, 60-min cumulative values were calculated. Both T3 and T4 inhibited insulin secretion and induced concomitant inhibitory effects on plasma cholesterol levels, a parameter of experimental hyperthyroidism. The correlation demonstrates that inhibition of glucose-induced insulin secretion from the pancreas is a specific effect of thyroid hormones. The inhibitory effect of T3 was five times greater than the inhibitory effect of T4. An excess of thyroid hormones induced hyperthyroidism, with its well-known increased incidence of diabetes.