In an attempt to compare the cholesterol-lowering effects of equivalent doses of d- and l-thyroxine, 10 euthyroid, hypercholesterolemic subjects were treated with graded doses of each medication in a cross-over design using thyrotropin suppression following thyrotropin-releasing hormone administration as the end-point. The mean thyrotropin-suppressive dose of d-thyroxine was 2.4 ± 0.66 mg per day, which resulted in mean reductions of 10 percent in total plasma cholesterol, 10 percent in plasma low-density lipoprotein cholesterol, and 11 percent in plasma high-density lipoprotein cholesterol. The mean thyrotropin-suppressive dose of l-thyroxine was 135 ± 46 μg per day, which resulted in mean reductions of 7 percent in total plasma cholesterol, 6 percent in plasma low-density lipoprotein cholesterol, and 14 percent in plasma high-density lipoprotein cholesterol. The reductions in total, low-density, and high-density cholesterol achieved with d-thyroxine were not significantly different from those achieved with l-thyroxine. Neither medication produced a significant increase in heart rate or ventricular ectopy as determined by Holter monitoring. These data do not support the belief that d-thyroxine has a preferential cholesterol-lowering effect in humans when compared with equivalent doses of l-thyroxine. In addition, both d- and l-thyroxine reduced plasma high-density lipoprotein cholesterol.
Bibliographical noteFunding Information:
From the Divisions of Endocrinology and Metabolism and Clinical Pharmacology, Department of Medicine, University of Minnesota, Minneapolis, Minnesota. This work was supported by National Institutes of Health Grant AM-19812, General Clinical Research Center Grant RR400 from the Division of Research Sources, National Institutes of Health, and a grant-in-aid from Travenol Laboratories. Requests for reprints should be addressed to Dr. John P. Bantle. Box 9 1, Mayo Memorial Building, University of Minnesota, Minneapolis, Minnesota 55455. Manuscript accepted March 26, 1984.