Alterations of serum high-density lipoproteins and hepatic lipase activity in congenital hypothyroidism.

Fourteen term newborns and infants with congenital hypothyroidism were studied before (T0) and after (T1) 1 month of replacement therapy. From T0 to T1 serum total cholesterol and high-density-lipoprotein cholesterol remained constant and at normal levels whereas low-density-lipoprotein apolipoprotein B and triacyglycerol concentrations increased slightly. The proportion of large-size high-density-lipoprotein subfraction was high at T0 and decreased very significantly at T1. Conversely, postheparin serum hepatic lipase activity was low before treatment and increased after thyroxine therapy. Lipoprotein lipase activity remained low throughout the study. These results suggest that, in contrast to adult hypothyroidism, low-density-lipoprotein catabolism is not altered in congenital hypothyroidism. However, as in adults, a defect of lipolytic enzyme activities is present and can induce an impairment of the elimination of cholesterol via the high-density-lipoprotein route and, potentially, of the catabolism of triglyceride-rich lipoproteins. Beyond the scope of congenital hypothyroidism, one should keep in mind these results when attending infants with transient low thyroid hormone levels and lipid intolerance.