PLASMA-LEVELS OF LIPOPROTEINS AND APOLIPOPROTEINS IN CONGENITAL HYPOTHYROIDISM - EFFECTS OF L-THYROXINE SUBSTITUTION THERAPY

Thyroid status in humans is an important factor in the regulation of lipoprotein metabolism. There are several data on hypothyroidism in the adult population, but less information is available about congenital hypothyroidism. Since lipid metabolism at birth is substantially different from that of adults, it is not likely that the same abnormalities that occur in adult hypothyroidism are also present when this is diagnosed at early life. We studied 16 subjects with congenital hypothyroidism, seven at the time of diagnosis and also after normalization of thyroid hormone levels over a period of 2.0 +/- 1.0 months of substitution therapy with L-thyroxine (5.9 +/- 1.2 mu g/kg/d) and nine already on L-thyroxine therapy for a period of 4.7 +/- 3.2 months. Thirty-nine apparently healthy subjects matched for age were selected as controls. In all subjects, total cholesterol (CHO), triglycerides (TG), low-density lipoprotein (LDL) cholesterol, high-density lipoprotein (HDL) cholesterol (HDL-C), apolipoproteins (ape) A-I and B, lipoprotein(a) [Lp(a)] thyrotropin (TSH), (LDL-C), total and free thyroxine (T-4), and triiodothyronine (T-3) were determined. CHO, HDL-C. and apo A-I revels were significantly higher in patients at the time of diagnosis than in controls (respectively, P=.0079, .0007, and .0004), whereas TG, LDL-C, apo B, and Lp(a) levels were not significantly different. During L-thyroxine substitution therapy in these subjects, HDL-C and apo A-I levels significantly decreased (respectively, by a mean of -36.2% and -24.4%), with similar behavior in all subjects. To verify if these lipoprotein changes were due to L-thyroxine substitution therapy or to physiological modifications with time, we remeasured lipoprotein parameters also in 10 normal subjects approximately 2 months later than the first measurement, but no significant change was found. Moreover, all subjects with congenital hypothyroidism on L-thyroxine substitution therapy did not show any significant difference in lipoprotein profile in comparison to age-matched controls. Our data document that infants with congenital hypothyroidism mainly show high HDL-C levels, and that L-thyroxine substitution therapy completely restores a normal lipid profile. Since HDL plays a major role in newborns and in the first days of life, we suggest that thyroid hormones modulate the physiological development of lipoprotein metabolism after birth, and that the lack of thyroid hormones does not permit complete evolution of newborn lipid patterns toward an adult profile. Copyright (C) 1995 by W.B. Saunders Company