Effect of levo-thyroxine replacement on non-high-density lipoprotein cholesterol in hypothyroid patients

Context: Recently, non-high-density lipoprotein cholesterol (non-HDL-C), a measure of total cholesterol minus HDL-C, has emerged as a predictor of cardiovascular disease. Objective: We evaluated the effect of L-T-4 replacement on non-HDL-C levels in patients with primary hypothyroidism. Methods: Thirteen patients with overt hypothyroidism and 26 patients with subclinical hypothyroidism participated in the study. The lipid profiles, including non-HDL-C, were measured in patients with hypothyroidism before and 3 months after L-T-4 replacement was started. Results: After L-T-4 replacement, the serum concentrations of all lipoproteins, exclusive of lipoprotein (a) [Lp(a)], were significantly decreased in patients with overt hypothyroidism. In patients with subclinical hypothyroidism, the serum concentrations of total cholesterol, non-HDL-C, remnant-like particle cholesterol, and apolipoprotein B (Apo B) were significantly decreased, whereas no significant changes in the serum concentrations of low-density lipoprotein cholesterol, HDL-C, triglycerides, apolipoprotein A-I, and Lp(a) were observed. In all 39 patients, the reduction in the non-HDL-C levels correlated with the reduction in the low-density lipoprotein cholesterol, remnant-like particle cholesterol, and Apo B levels. However, the reduction in the non- HDL-C levels did not correlate with the reduction in the HDL-C, Lp(a), and apolipoprotein A-I levels. Conclusions: This study is the first to show that L-T-4 replacement may reduce serum concentrations of non- HDL-C in patients with hypothyroidism. The study also suggests that such altered serum concentrations of non- HDL-C in hypothyroidism may be related to the disturbed metabolism of low-density lipoprotein, remnant lipoprotein, and Apo B.