Thyroid hormones inhibit growth hormone secretion in domestic fowl (Gallus domesticus).

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 micrograms/day for 10 days or 250 micrograms/kg for 7 days) or T3 (250 micrograms/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 micrograms/kg) or T4 (250 micrograms/kg). In control birds the concentration of plasma GH was greatly increased (greater than 450 micrograms/l) within 10 min of an i.v. injection of thyrotrophin releasing hormone (TRH; 10 micrograms/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.