Tissue distribution of thyroxin analogues and their effects on cholesterol metabolism

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Abstract
Four thyroxin analogues, l-triiodothyronine, d-triiodothyronine, tetraiodothyropropionic acid, and tetraiodothyroformic acid, labeled with I131, were injected intravenously into male rats and the tissue distributions of radioactivity determined at intervals of 40 min, 2 hr, and 6 hr. Of the two analogues that have been shown to exert an effect on cholesterol metabolism, which is accentuated relative to their other metabolic activities, d-triiodothyronine and tetraiodothyroformic acid, only the former was disproportionately concentrated by the liver. The liver to skeletal muscle concentration ratio of tetraiodothyroformic acid did not differ appreciably from those of l-triiodothyronine and tetraiodothyropropionic acid, which do not display an accentuated effect on cholesterol metabolism. It is concluded that the effect on cholesterol metabolism relative to other metabolic activities of thyroxine analogues is not determined solely by their tissue distributions.