Thyrotropin-releasing hormone secretory physiology: studies by radioimmunoassay and affinity chromatography

Thyrotropin-releasing hormone (TRH) in rat blood and mathanol extracts of hypothalami has been quantified by a specific TRH radioimmunoassay, in conjunction with TRH affinity chromatography. Blood TRH was found to be elevated dramatically after 1 h by the physiologic stimulus, ambient cold (4 C), which was also attended by a 2-fold rise in serum thyrotropin (TSH). The acute administration of parenteral triodothyronine (T3) over a period of 48 h prior to identical cold exposure completely abolished the rise in serum TSH. Blood TRH however, rose in a fashion qualitatively similar to that seen in euthyroid animals. Similarly, chronic induction (32 days) of experimental euthyroidism, hypothyroidism, and hyperthyroidism, which altered metabolic states were corroborated by measurement of serum thyroxine (T4) and TSH, did not result in any significant changes in either the hypothalamic content or blood concentration of TRH. These data provide direct documentation that the regulatory control of pituitary TSH secretion by T4 and/or T3 is not mediated by inhibiting the hypothalamic secretion of TRH, but presumably by inhibiting the actions of TRH at the level of the pituitary thyrotroph cell.