The Relative Potencies of Thyroxine and Liothyronine by Oral and Subcutaneous Administration in the Rat

Abstract

Abstract In goitre prevention assays in adult female rats, thyroxine was only one-fifth to one-sixth as active by mouth as by subcutaneous injection whereas liothyronine had about the same biological activity for the two routes of administration. These findings provide an explanation for the fact that thyroxine was one-sixth to one-seventh as active as liothyronine when injected subcutaneously but only one-twentieth to one-thirtieth as potent when the hormones were administered orally. Chemical examination of three commercially available thyroxine samples which were labelled as “chromatographically pure” revealed that one of them contained approximately 10 per cent liothyronine. The presence of this contaminant had a marked influence on the biological responses of the test animals to this preparation.