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The influence of thyroxine (T₄) and tri-iodothyronine (T₃) on the secretion of GH in immature fowl was investigated. In birds pretreated with i.m. injections of T₄ (100 µg/day for 10 days or 250 µg/kg for 7 days) or T₃ (250 µg/kg for 7 days) the basal plasma GH level was markedly reduced. A similar reduction in the basal plasma GH level was also observed 60 min after a single injection or T₃ (25 and 250 µg/kg) or T₄ (250 µg/kg).

In control birds the concentration of plasma GH was greatly increased (> 450 µg/l) within 10 min of an i.v. injection of thyrotrophin releasing hormone (TRH; 10 µg/kg). In birds pretreated with T₃ or T₄ the increase in GH concentration after TRH treatment was significantly less than that in the controls. In birds pretreated for 60 min with T₃ or T₄ the GH response to TRH was inversely dose-related and lowest in T₃-treated birds. These results demonstrate that T₃ and T₄ inhibit GH secretion in birds, which is an effect not observed in mammalian species.