Phenobarbital injection reduces the biologic effectiveness of thyroxine in thyroidectomized rats maintained on a constant daily dose of hormone. This was inferred from measurements of total body oxygen consumption and the activity of the mitochondrial enzyme alpha-glycerophosphate dehydrogenase in liver, heart and kidney. No changes in these parameters were observed when intact rats were given phenobarbital alone. Thus, the increased hepatocellular binding and deiodination of T4 stimulated by phenobarbital is not related to the biologic action of the hormone. On the contrary, phenobarbital administration reduces the biologic effectiveness of administered thyroxine by diverting hormone from extrahepatic and from specific intrahepatic sites of action.
ASJC Scopus subject areas