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

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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.