THYROXINE-LIKE COMPOUNDS AND CHOLESTEROL METABOLISM: DIFFERENCES IN THE EFFECTS OF THYROXINE, TRIIODOTHYRONINE AND THEIR FORMIC ACID ANALOGUES1

Abstract

The biologic activity of l–thyroxine, l–triiodothyronine, and their formic acid analogues, tetraiodothyroformic acid and triiodothyroformic acid have been studied in the rat. By thiouracil-goitre inhibition assay they were found to have the following relative activities (on a molar basis): l-thjToxine, 100%; l–triiodothyronine, 333%; tetraiodothyroformic acid, 0.08% and triiodothyro-formic acid 0.27%. In addition to this quantitative difference, the physiologic activity of tetra-iodothyroformic acid was also found to differ in other respects from that of l–thyroxine. When administered in amounts having equal goitre inhibiting ac-tivity, the formic acid analogue exerted significantly greater effect on cholesterol metabolism than didl–thyroxine. As compared to l–thyroxine, tetraiodothyro-formic acid also exerted relatively greater effect on cholesterol metabolism than on growth.