Interrelation of Insulin Activity and Thyroid Function

¹ From the Department of Medicine, Harvard Medical School; the Medical Research Department, Yamins Research Laboratories and the Medical Service, Beth Israel Hospital, Boston, Massachusetts

The effect of variation in thyroid status on insulin metabolism and glycogen synthesis has been studied in rats. In vitro incubation of rat diaphragm with 0.1 mg triiodothyronine or 0.1 mg thyroxine for 30 and 45 minutes had no effect on I¹³¹ insulin uptake of the diaphragm. Daily intraperitoneal injections of 50 triiodothyronine increased the oxygen consumption and had an increasing effect on in vitro insulin diaphragm uptake, beginning after 2 days, reaching a maximum of 32% over controls on the 5th day. With continued administration of triiodothyronine diaphragm insulin uptake declined to control levels on the twelfth day of triiodothyronine administration. During the first 2 minutes after intravenous administration of I¹³¹ insulin in triiodothyronine treated animals (50 daily, i.p. for 5 days) muscle showed an increased uptake of insulin. There was a subsequent progressive fall in tissue uptake to approximately levels found in control animals. Blood radioactivity disappeared more rapidly after injection in the thyrotoxic animal than in the control or hypothyroid rats. Glycogen deposition (insulin-glycogen effect) in the thyrotoxic animals was greater than in controls. However, while tissues from control animals continued to deposit glycogen at the same rate, the diaphragm from thyrotoxic rats showed after 30 minutes of incubation a marked fall in glycogen synthesis. Glycogen synthesis by diaphragm from triiodothyronine treated animals incubated in glucose alone without insulin equilibration was the same as control diaphragm.