Abstract A 58-year-old patient with heterozygous familial hypercholesterolemia (FH) showed normal levels of serum cholesterol (193 mg/dL) in coexistence with hyperthryoidism. After hyperthyroidism therapy with radioiodine and methimazole, the patient's lipid profile showed high concentrations of cholesterol (whole serum 318 mg/dL, VLDL 35 mg/dL, LDL 217 mg/dL, HDL 44 mg/dL). There was a significant inverse correlation between serum cholesterol levels and serum thyroxine levels ( r = −0.815, p < 0.01). Effects of triiodothyronine on LDL degradation and cholesterol synthesis from 14C-labeled acetate were studied in cultured skin fibroblasts. Triiodothyronine (T 3) stimulated both LDL degradation and cholesterol synthesis in the cells from normal subjects and patients with heterozygous FH. The T 3 increased cellular cholesterol synthesis markedly in the cells from patients with homozygous FH but did not increase LDL receptor activity. These results suggest that normal serum cholesterol levels in our case result in part from an enhancement of LDL receptors by thyroid hormone.