Effects of Clofibrate (Atromid S) on the Thyroxine-Binding Capacity of Thyroxine-Binding Globulin and Free Thyroxine

Abstract

Estimations of the maximum thyroxine binding capacity of thyroxine-binding globulin (TBG) and of the free thyroxine fraction were undertaken in 12 patients with hyperlipoproteinemia, comprising 9 males and 3 females, who were receiving the hypocholesterolemic drug clofibrate (Atromid S). All patients were placed on an isocaloric diet for 3 months, which commenced 1 month prior to the initiation of 2 monthly periods of study. In one period patients received clofibrate and in the other a placebo of identical appearance. The periods were randomly distributed among the patients a double blind study. Determinations of the thyroxine-binding capacity of TBG and the percentage free thyroxine were carried out twice during each of the 2 periods. The results show a significant (paired t-test, p<0.01) elevation in the mean maximal thyroxine-binding capacity of TBG in 11 patients and a small but significant fall in the mean percentage free thyroxine in 9 patients while they were receiving clofibrate. The free thyroxine iodine concentration was calculated in each patient both on and off clofibrate therapy, and no significant difference between the 2 periods was observed. In experiments in which a sodium salt of clofibrate was added directly to serum no change in TBG binding capacity but a small rise in percentage free thyroxine was observed. The changes observed in TBG and percentage free thyroxine bear no relationship to the lipid response to clofibrate in the patients studied. It is unlikely that the lipidlowering effect of this drug can be directly explained in terms of these changes.