Endogenous growth hormone controls high plasma levels of 3,3′,5-triiodothyronine (T 3) in growing chickens by decreasing the T 3-degrading type III deiodinase activity

Abstract

The influence of endogenous GH levels on peripheral monodeiodination activity has been investigated in growing chickens at the age of 4 weeks, when they normally show no T 3 increase after GH injection. Injection of anti-GH serum decreased plasma T 3 and increased plasma T 4. Three d and 1 week after hypophysectomy, plasma T 3 was also markedly decreased, while T 4 was only slightly affected, hepatic 5′D-I activity showed a transient decrease, but 5D-III activity was highly increased, as were the number of hepatic GH receptor sites. Injection of GH in hypophysectomized chickens decreased 5D-III activity and increased plasma T 3. GH receptor-deficient dwarf chickens had decreased plasma T 3 and increased plasma T 4 and hepatic 5′D-I and 5D-III activities compared to their normally-growing siblings. GH administration could only affect T 3 and 5D-III in the non-dwarf siblings, which showed higher basal 5D-III activity compared to the non-responsive age-matched chickens of the Hisex strain used in the other experiments. It can be concluded that endogenous GH is an important factor in the control of plasma T 3 levels in growing chickens due to its influence on the activity of the T 3-degrading type III deiodinase. The effectiveness of exogenous GH administration to acutely increase plasma T 3 probably depends on the balance between the injected dose and the endogenous GH concentration, the hepatic GH receptor availability and the hepatic type III deiodinase level.