Heritability of variation of plasma cortisol levels

Heritability of the variation of the plasma concentrations of total and unbound cortisol, cortisol binding globulin (CBG), and dehydroepiandrosterone sulfate (DHEA-S) was investigated in 20 monozygotic (MZ) and 20 dizygotic (DZ) male twin pairs. Three plasma samples collected between 8 AM and 9:30 AM were pooled for the assays. Heritability was calculated from the intraclass correlation [2(rMZ - rDZ)]. The mean age, total and unbound cortisol, CBG, and DHEA-S were not significantly different between the MZ and DZ groups of twins. The heritability index for variability of the plasma content of steroids was 45.4% (p less than .05) for total cortisol, 50.6% (P less than .05) for unbound plasma cortisol, 57.8% (P less than .05) for DHEA-S, and 32.4% (P greater than .05) for CBG. The data were analyzed by factor analysis, and heritability estimates were corrected for factors including age, smoking, drinking, exercise, and degree of obesity. These factors did not account for the variation in hormone values in twin pairs. Factor analysis of the three quantitative measurements, cortisol, percent free cortisol, and DHEA-S, provides no evidence for shared factors. The correlation coefficients between age and CBG and total and unbound plasma cortisol concentrations were insignificant. The correlation coefficient between total plasma cortisol levels and CBG was 0.57, which indicates that CBG accounts for 32% of the variation of plasma cortisol concentrations. The results suggest that genetic factors have a decided influence on the variation of the concentration of cortisol of DHEA-S in normal adult men.