The relationship between lipoprotein(a) and low density lipoprotein receptors during the treatment of hyperthyroidism.

To determine whether the low density lipoprotein (LDL) receptor pathway is involved in the catabolism of plasma lipoprotein (a) [Lp(a)], serum lipids, Lp(a), and LDL receptor activity were measured in seven patients with hyperthyroidism before and after methimazole treatment given hyperthyroidism is associated with enhanced LDL receptor activity. LDL receptor activity in patients was estimated by the equation using the serum concentrations of apolipoprotein (apo) B and C-II. When euthyroidism was achieved after treatment, not only did serum total cholesterol, high density lipoprotein-cholesterol, apo B, and LDL-cholesterol (LDL-Ch) levels rise, but Lp(a) significantly increased and calculated LDL receptor activity significantly decreased. The changes in LDL receptor activity were significantly correlated with the changes in LDL-Ch as expected, but not with changes in Lp(a). These results suggest that the serum concentration of Lp(a) is lowered in hyperthyroidism, probably by a mechanism other than the enhanced activity of the LDL receptor, and that the LDL receptor pathway is involved in the catabolism of Lp(a) to a limited extent.