Inhibition of Ovine Prolactin and Thyrotropin Responses to Thyrotropin-Releasing Hormone by Triiodothyronine

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Abstract
Studies were conducted with mature crossbred ewes to determine the influence of triiodothyronine (T3) on the pituitary secretion of prolactin (PRL) and thyrotropin (TSH). Intravenous injection of T3 (50 µg) 1 hr prior to thyrotropin releasing hormone (TRH) treatment resulted in plasma T3 concentrations of 24 ng/ml as compared to 1.5 ng/ml in control animals. Plasma T3 concentrations after intravenous injection of 50 µg decreased with a half life of 1 hr 39 minute. Acute T3 treatment completely abolished (P<.01) the TSH secretion and reduced (P<.05) the PRL secretion in response to TRH. Plasma T3 concentrations 6 hr after the final T3 injection in chronically treated ewes (50 µg subcutaneously twice daily for 2 weeks) were 3 ng/ml as compared to 1.5 ng/ml in control ewes. This twofold elevation in plasma T3 reduced (P<.05) both basal plasma TSH and TRH stimulation of TSH secretion. Neither basal nor TRH stimulated plasma PRL concentrations were influenced by the chronic T3 treatment. Injection of TRH (50 µg) into control ewes resulted in a subsequent increase in endogenous plasma T3 concentrations from 1.5 to 3.1 ng/ml. These results suggest that normal plasma concentrations of T3 are probably not involved in physiologic regulation of PRL secretion. However, very high plasma concentrations of T3 are capable of inhibiting the stimulation of PRL and TSH secretion by TRH, presumably at the level of the anterior pituitary. Copyright © 1976. American Society of Animal Science . Copyright 1976 by American Society of Animal Science.

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