The composition of low-density lipoprotein and very-low-density lipoprotein subfractions in primary hypothyroidism and the effect of hormone-replacement therapy.

1. The lipid and protein composition of subfractions of plasma low-density lipoprotein (LDL) has been determined in nine patients with primary hypothyroidism before and after 3 months of thyroxine therapy. Analyses were also made of subfractions of very-low-density lipoprotein (VLDL) in four of the patients. 2. Before therapy seven of the patients had elevated LDL-cholesterol and two had increased VLDL-cholesterol concentrations. On thyroxine replacement the mean LDL-cholesterol fell to normal. No significant change occurred in the concentration of cholesterol in VLDL or in highdensity lipoprotein (HDL). 3. The concentrations of cholesterol, triglyceride and apolipoprotein B (apoB) were increased in the LDL subfraction of S, 10-4-20, which corresponds mainly to intermediate-density lipoprotein. This subfraction showed a marked fall on therapy. The cholesterol and apoB concentrations in the major LDL fraction of S, 5.7-12 also decreased on therapy, but the fall in the subfraction of S, 3.56-5 did not reach statistical significance. 4. Only the VLDL subfraction of smallest size (S, 20-60) had any abnormality before therapy, with an increased concentration of cholesterol. On thyroxine the concentration of triglyceride rose in the VLDL subfractions.