PARTICIPATION OF TRI-IODOTHYRONINE AND METABOLIC-CLEARANCE RATE IN THE INHIBITION OF GROWTH-HORMONE SECRETION IN THYROXINE-TREATED DOMESTIC-FOWL

Surgical thyroidectomy increases basal and TRH-induced GH concentrations in the peripheral plasma of immature domestic fowl. Replacement therapy with thyroxine (T4; 100 μg/kg per day for 7 days, i.m.) suppressed the GH responses to thyroidectomy. Bolus administration of T4 (10 μg/kg, i.m.) to thyroidectomized birds promptly lowered the circulating GH concentrations, which remained suppressed for at east 4 h. Chronic (daily injections for 7 days) or acute (one injection) pretreatment of thyroidectomized birds with iopanoic acid (IOP; 40 mg/bird, i.m.) before the bolus administration of T4 attenuated, but did not prevent, inhibition of circulating GH levels by T4. Administration of IOP (40 mg/bird i.m.) 24 h and immediately before the administration of tri-iodothyronine (T3; 3 μg/kg, i.m.) or T4 (10 μg/kg, i.m.) also failed to suppress thyroidal inhibition of circulating GH concentrations in thyroidectomized birds. Administration of IOP alone had no effect on GH concentrations. Circulating T3 concentrations were not enhanced following the administration of T4 to IOP-treated birds, indicating is inhibition of hepatic monodeiodinase activity. The metabolic clearance rate (MCR) of 125I-labelled chicken GH in the plasma of thyroidectomized fowl was less than that in sham-thyroidectomized birds. Following pretreatment with T4 (100 μg/kg per day for 7 days) sham-thyroidectomized and thyroidectomized birds did not differ significantly in their MCR. The GH secretion rate in thyroidectomized birds was similar to that in sham-thyroidectomized birds and in both groups was markedly reduced following pretreatment with T4. These results demonstrate thyroidal inhibition of circulating GH concentrations in fowl. Both T3 and T4 inhibited GH concentrations and the effect of T4 was not simply due to its role as a T3 prohormone. In the absence of thyroid hormones, the MCR of GH was reduced but its secretion rate was not enhanced. A significant reduction of GH secretion rate occurs in response to exogenous T4, in the absence of any change in GH metabolism.