Glucose tolerance and pancreatic hormones in thyroidectomized and thyroid hormone-injected cockerels

The effect of surgical thyroidectomy and of T4/T3 injections on basal and glucose-induced concentrations of plasma insulin and glucagon has been investigated in 20-week-old domestic chickens. Birds injected daily (im) for 2 weeks with T4/T3 (50 micrograms/day) had marginally lower fasting glucose concentrations whereas thyroidectomy had no effect. Glucose tolerance to an intravenous injection of glucose (0.5 g/kg) was impaired in T4/T3 injected animals although the peak hyperglycemia was identical with sham-operated animals. This was associated with significantly reduced basal and glucose-induced insulin concentrations. However, fasting plasma glucagon concentrations were significantly elevated in this group as was the magnitude of the glucose-induced suppression of glucagon release 10 min after injection (48% decline vs 34% in sham-operated animals). Basal concentrations of plasma insulin were markedly elevated in thyroidectomized animals and were associated with only mildly depressed plasma glucagon levels. The absolute concentrations of plasma insulin remained higher in the thyroidectomized birds as compared with those of sham-operated or T4/T3 injected animals after the glucose challenge, although within 30 min after glucose injection they had significantly declined below preinjection levels. This was associated both with significantly reduced plasma glucose concentrations 30 min after injection and the lowest absolute levels of plasma glucagon. The rebound in plasma glucagon in sham-operated animals in response to the rapid decline in glucose concentrations was not as pronounced in either thyroidectomized or T4/T3 injected animals. In conclusion these studies illustrate the secretory dynamics of avian pancreatic endocrine islets in response to both absolute glucose levels and glucose requirements as affected by the thyroid state of the bird.