New Assay Procedure for Thyroxine Analogs

Abstract

The large increase in rat liver and kidney [alpha]-glycerophosphate dehydrogenase activity (GPD) which results from the administration of thyroxine (T4) has been developed into a simple assay procedure for thyromimetic substances. The dose-response curves obtained with T4 and other analogs were analyzed by 2 variations of the usual semilog plot as well as by a new hemisigmoid procedure. The results agreed well with previously reported calorigenic activities. Both weanling and adult rats produced endogenously the equivalent of about 1 [mu]g T/100 g body wt./day, but the adults were appreciably less sensitive to the hormone. The nature of the diet fed to rats during the assay period modified the response even when the T4 was administered subcutaneously; approximately 3 times as much T4 was required to produce a given response on a chow diet as on a purified diet.