Relationship of volumetric bone density and structural parameters at different skeletal sites to sex steroid levels in women

Context: Although estrogen clearly plays a central role in regulating bone mass in women, studies in men have suggested that there may be a threshold bioavailable (bio) estradiol (E-2) level below which aging men begin to lose bone and that the threshold for estrogen deficiency in cortical bone may be considerably lower than that in trabecular bone. There are no data testing this in women. Objective: Our objective was to assess volumetric bone mineral density (vBMD) and bone geometry by quantitative computed tomography and relate these to circulating bio E-2 and bio testosterone levels. Design: We studied a cross-sectional, age-stratified population sample of 235 women (age, 21-97 yr). Results: vBMD/structural parameters were not related to sex steroid levels in young premenopausal women (age, 20-39 yr) with a median bio E-2 level of 17 pg/ml (63 pmol/ liter). By contrast, bio E-2 and bio testosterone levels were both significantly associated with trabecular and cortical vBMD and cortical area at multiple sites in late postmenopausal women (age >= 60 yr) who had a median bio E-2 level of 3 pg/ml (11 pmol/ liter). Late premenopausal and early postmenopausal women ( age, 40 - 59 yr) with an intermediate median bio E-2 level of 11 pg/ ml (42 pmol/ liter) showed age-adjusted correlations of bio E-2 levels with trabecular but not with cortical vBMD. Conclusions: In women, bio E-2 levels are associated with vBMD and some structural bone parameters at low but not high bio E-2 levels. Similar to findings in men, the threshold for estrogen deficiency in cortical bone in women appears to be lower than that in trabecular bone.