Cholesterol 7αHydroxylase Activity in Hypothyroidism and Hyperthyroidism in Humans

Alterations of serum cholesterol levels are well recognized findings in hypothyroidism and hyperthyroidism. It remains unclear, whether thyroid hormones may affect serum concentrations of cholesterol through changes in the activity of cholesterol 7α-hydroxylase, the rate-limiting enzyme in the catabolic conversion of cholesterol to bile acids. We determined serum concentrations of the bile acid precursor 7α-hydroxy-4-cholesten-3-one, which reflects cholesterol 7α-hydroxylase activity in the liver, in 19 patients with hypothyroidism and in 10 patients with hyperthyroidism before and after treatment, respectively. In patients with hypothyroidism, serum concentrations of cholesterol and LDL-cholesterol decreased by 33% (p < 0.0005) and 39% (p < 0.0005), respectively, after replacement therapy with thyroid hormones. In contrast, serum concentrations of 7α-hydroxy-4-cholesten-3-one (21.7 ± 15.8 ng/ml vs 24.5 ± 18.1 ng/ml before treatment, n.s.) as well as serum HDL-cholesterol were unchanged during substitution therapy. In patients with hyperthyroidism, serum concentrations of cholesterol and LDL-cholesterol increased by 27% (p < 0.01) and 39% (p < 0.01) after antithyroid treatment, respectively. Again, serum concentrations of 7α-hydroxy-4-cholesten-3-one did not change significantly during treatment (15.8 ± 12.6 ng/ml vs 14.7 ± 8.1 ng/ml before treatment, n.s.). These findings indicate that in humans, thyroid hormones influence serum lipid concentrations by other mechanisms than by affecting the activity of cholesterol 7α-hydroxylase.

Key words

Cholesterol 7α-Hydroxylase - Hypothyroidism - Hyperthyroidism