Changes in serum apolipoprotein concentrations after L-thyroxine therapy in infants with congenital hypothyroidism.

To further characterize the impact of thyroid hormones on the serum lipid profile, we studied serum apolipoproteins in infants with congenital hypothyroidism before and after L-thyroxine (L-T4) replacement therapy. Serum high-density lipoprotein cholesterol (HDLC) decreased after L-T4 therapy. Total cholesterol (TC) and low-density lipoprotein cholesterol (LDLC) levels did not change significantly after therapy. Two months after L-T4 replacement therapy, serum apolipoprotein A-I (apo A-I), C-III, and E declined and apo B increased significantly. No significant changes were observed for serum concentrations of apo A-II and C-II after L-T4 substitution. We conclude that in infants, thyroid hormone reduces serum levels of apo A-I, the principal protein component of HDLC, and this may contribute to the decline of serum HDLC concentrations after L-T4 replacement therapy.