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The number, but not affinity, of binding sites for [³H]3-methyl-histidine²-TRH ([³H]Me-TRH) on chicken adenohypophysial plasma membranes was increased in chickens made hypothyroid by goitrogen (methimazole) treatment (50 mg/kg per day for 7 days), which also increased circulating GH concentrations. Daily i.p. injection of thyroxine (T₄; 100 µg/kg for 7 days) had no effect on [³H]Me-TRH binding to pituitary membranes, although it suppressed endogenous GH secretion. Binding of [³H]Me-TRH to pituitary caudal lobe membranes was, however, suppressed by tri-iodothyronine (T₃) injected chronically (100 µg/kg per day, i.p., for 7 days) or acutely (100 µg/kg, 2 h before being killed). The suppression of [³H]Me-TRH binding and inhibition of GH secretion following T₃ administration was dose related. Binding of [³H]Me-TRH to caudal lobe membranes was also suppressed following the incubation of pituitary glands with T₃ in vitro, and the response was both dose and time related.

These results suggest that T₃ inhibits GH secretion in fowl by a down-regulation of pituitary TRH receptors. However, other mechanisms are involved in thyroidal inhibition of GH release in birds, since T₄ had no effects on [³H]Me-TRH binding yet suppressed GH secretion in vivo.