Influence of Chronic Thyroxine Treatment on Plasma Hormone and Metabolite Concentrations and on Responses to Insulin, Glucagon and Thyrotrophin Releasing Hormone in Adult Sheep

Summary

Four adult sheep fed twice daily were given daily subcutaneous injections of saline for four weeks, followed by a similar period of daily L-thyroxine (T₄) injection (1 mg/day). T₄ treatment increased basal plasma concentrations of T₄, triiodothyronine (T₃), insulin and glucose, together with T₃-uptake and the free thyroxine index, while cholesterol and urea concentrations decreased.

T₄ treatment reduced the rise in prolactin levels after the morning meal. Thyrotrophin releasing hormone (TRH) injection increased plasma T₃ only in the control period and T₃-uptake only in the T₄ treatment period. T₄ treatment did not affect the prolactin response to TRH injection or the insulin and glucose responses to glucagon injection. The increase in insulin concentrations after insulin injection and the secondary hyperglycaemia following initial insulin-induced hypoglycaemia were reduced by T₄ treatment.

Key-Words:

Hyperthyroidism - Thyroxine - Triiodothyronine - Thyrotrophin Releasing Hormone - Glucagon - Insulin - Prolactin - Glucose - Sheep