Growth Hormone Induced Stimulation of the T₄ to T₃ Conversion in Fed and Fasting Dwarf Goats

Summary

Fed and food deprived (7 days) adult dwarf goats were injected intravenously with 75 μg/kg b. w. of ovine growth hormone (o-GH). Blood samples were collected from the jugular vein -2, -1 and 0 hr prior to and 30, 60, 90 and 120 min after injection and assayed for thyroxine (T₄) and triiodothyronine (T₃) concentrations. The 5'-monodeiodination (5'-D) activity was consequently determined in liver and kidney samples following slaughtering. Fasting alone increased plasma concentrations of T₃ and T₄, whereas injection of o-GH raised T₃ additionally and more profoundly in food deprived animals compared to fed ones. A small increase in plasma T₄ was also observed following o-GH injection, but only in starved goats. At the same time the hepatic, but not the kidney, 5'-D activity was stimulated in food deprived and GH injected animals. It is concluded that during prolonged fasting an increased peripheral T₄ to T₃ conversion is occurring contrary to the known decrease in T₃ production during short periods of food deprivation. This increased conversion may be under the control of GH.

Key words

Fasting - Thyroid - GH in Goats