Activity-structure relationships of the thyrotropin releasing hormone

Abstract Analogs of the thyrotropin releasing hormone (TRH) have been studied for activity. Although the structure of TRH is highly specific for activity, some analogs released TSH at levels of about 20 to 10,000 times that of TRH. A similarity in the activity of the analogs to that of TRH is indicated by the inhibition in vivo by triiodothyronine and their activity in vitro . TRH, but not the active analogs, was inactivated in human serum showing a specificity of the inactivation. The in vivo activity of an analog at microgram levels is that of a relatively potent substance although these levels are up to 10,000 times higher than those of TRH; TRH has extraordinary potency. The potency of some of these analogs may be due to no inactivation by serum.