Associations among oral estrogen use, free testosterone concentration, andlean body mass among postmenopausal women

Circulating concentrations of sex hormone-binding globulin (SHBG) are incre ased by use of oral estrogen. The objective of this study was to determine whether postmenopausal women who used oral estrogen had higher serum concen trations of SHBG and lower serum concentrations of free testosterone (T) th an nonusers, and whether free T was associated with lean body mass, particu larly skeletal muscle mass. Subjects were 70 postmenopausal women, 46-55 yr old, 46 of whom used oral estrogen. Total and regional body composition we re determined by dual-energy x-ray absorptiometry. Serum concentrations of SHBG, total T, and estradiol (E-2) were determined by RIA. Free T was calcu lated from concentrations of total T and SHBG. Hormone users had higher ser um concentrations of E-2 and SHBG (182.0 +/- 58.5 vs. 82.9 +/- 41.1 nmol/L, mean +/- SD, P < 0.001) and lower concentrations of free T (3.7 +/- 2.2 vs . 7.9 +/- 4.1 pmol/L, mean SD, P < 0.001); total T did not differ. Total le an mass and leg lean mass were significantly correlated with free, but not total T [r values of 0.29 (P < 0.05) and 0.31 (P < 0.01) for total and leg lean mass, respectively, vs. free T]; arm lean mass was not correlated with either measure of T. Serum E-2 was significantly correlated with SHBG (r = 0.50, P < 0.001) and free T (r -0.33, P < 0.01). These observations imply that, by reducing the concentration of bioavailable T, oral estrogen therap y may accelerate or augment lean mass loss among postmenopausal women. This conclusion awaits confirmation by longitudinal observation.