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 7 alpha-hydroxylase, the rate-limiting
enzyme in the catabolic conversion of
cholesterol to
bile acids. We determined serum concentrations of the
bile acid precursor
7 alpha-hydroxy-4-cholesten-3-one, which reflects
cholesterol 7 alpha-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
7 alpha-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
7 alpha-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 7 alpha-hydroxylase.