THE RELATIONSHIP BETWEEN LIPOPROTEIN(A) AND LOW-DENSITY-LIPOPROTEIN RECEPTORS DURING THE TREATMENT OF HYPERTHYROIDISM

To determine whether the low density lipoprotein (LDL) receptor pathwa y is involved in the catabolism of plasma lipoprotein (a) [Lp(a)], ser um lipids, Lp(a), and LDL receptor activity were measured in seven pat ients with hyperthyroidism before and after methimazole treatment give n hyperthyroidism is associated with enhanced LDL receptor activity. L DL receptor activity in patients was estimated by the equation using t he serum concentrations of apolipoprotein (ape) B and C-ll. When euthy roidism was achieved after treatment, not only did serum total cholest erol, high density lipoprotein-cholesterol, apo B, and LDL-cholesterol (LDL-Ch) levels rise, but Lp(a) significantly increased and calculate d LDL receptor activity significantly decreased. The changes in LDL re ceptor activity were significantly correlated with the changes in LDL- Ch as expected, but not with changes in Lp(a). These results suggest t hat the serum concentration of Lp(a) is lowered in hyperthyroidism, pr obably by a mechanism other than the enhanced activity of the LDL rece ptor, and that the LDL receptor pathway is involved in the catabolism of Lp(a) to a limited extent.