Antithyroxine effects of some thyroxine analogues

¹ Department of Pharmacology, University of Alabama Medical Center, Birmingham, Alabama

Several diiodo- and triiodothyronines and their propionic and acetic acid analogues were evaluated for thyroxine-antagonizing capacity. The postabsorptive metabolic rate of thyroidectomized rats maintained on 20 µg L-thyroxine/ kg/day was used as the criterion, to avoid involvement of endogenous thyroid hormone secretion. At an inhibitor to thyroxine molar ratio of 100:1, 3,3',5'-triiodothyronine, 3,3',5'-triiodothyropropionic acid, 3,3'-diiodothyronine and 3,3'-diiodothyropropionic acid completely suppressed the thyroxine effect on metabolic rate. At the same molar ratio, the corresponding diiodo- and triiodothyroacetic acids produced only about 50% inhibition.