Thyroxine inversely regulates serum intermediate density lipoprotein levels in children with congenital hypothyroidism

Background: Although there have been numerous studies on the effects of thyroid hormones on serum lipid profiles, the effects of thyroxine on intermediate low density lipoprotein (IDL) remain uncertain. In an attempt to clarify this issue, under conditions with very little influence exerted by sex hormones on serum lipid profiles, we studied the relationship between serum thyroid hormone levels and the proportion of serum IDL fractions in children. Methods: Nineteen children with congenital hypothyroidism and 13 children with non‐thyroid diseases were enrolled in this study. Blood samples were taken to measure serum thryoid stimulating hormone, triiodothyronine, free thyroxine (FT4), total cholesterol, high density lipoprotein (HDL) cholesterol, triglyceride and apolipoprotein levels. Lipoprotein fractions, including very low density lipoprotein (VLDL), IDL, low density lipoprotein (LDL) and HDL, were determined by their electrophoretic mobility in a non‐denaturing polyacrylamide gel. Results: The proportion of IDL fractions showed a significant inverse correlation with serum FT4 levels and a significant correlation with serum total cholesterol and apolipoprotein B and C‐II levels. Serum VLDL, LDL and HDL fractions did not correlate with serum thyroid hormone levels. Conclusion: From these results and other studies, we suggest that thyroxine promotes the conversion of IDL into LDL, possibly by its stimulatory effects on hepatic lipase activity.