Several relative biological activities of d(–) and l(+) triiodothyronine

¹ Research and Development Division, Smith Kline and French Laboratories, Philadelphia, Pennsylvania

d(–) 3,5,3'-Triiodothyronine administered subcutaneously to intact male rats for 10 days was found to be 0.25 as calorigenic as l(+) 3,5,3'-triiodothyronine. A dose of 15 µg/kg elevated the oxygen consumption of thyroidectomized rats to a level comparable to that obtained for the intact controls. When administered subcutaneously to thiouracil-treated rats d(–) 3,5,3'-triiodothyronine exhibited 0.08 the antigoitrogenic activity of l(+) 3,5,3'-triiodothyronine. A parallel regression analysis of the results obtained from plasma total cholesterol-lowering tests using cholesterol-fed rats or ethyl linoleate-fed mice indicates the need for considering the experimental design when interpreting cholesterol-lowering activity. d(–) 3,5,3'-Triiodothyronine was found to be 0.5 as active as its l(+) isomer in the rat plasma cholesterol-lowering test, while only 0.095 as active in the mouse procedure. Neither agent altered the liver cholesterol of treated mice.