Comparison of effectiveness of thyrotropin-suppressive doses of D- and L-thyroxine in treatment of hypercholesterolemia

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 micrograms 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.