Bone density of normal women in relation to endogenous and exogenous oestrogens

To evaluate the relationship between duration of hormone replacement therapy (HRT) and forearm bone density, a population-based study has been conducted of 4638 women comparing bone density of 1079 women who had taken HRT for varying times with those who had never taken it. Forearm bone mineral content (BMC) was measured by single photon absorptiometry. Additionally, free oestradiol and sex hormone binding globulin levels were measured in women with high and low bone mineral content. There were no detectable differences in BMC in those who took HRT for up to 24 months compared with those who did not. Thereafter there were significant positive regression coefficients for duration periods of 2 to 5 yr (beta = 0.77, P = 0.04 and beta = 1.76, P = 0.03) and more than 5 yr (beta = 1.66, P < 0.0005 and beta = 1.87, P = 0.009) in post-menopausal and oophorectomized women. The major determinants of BMC were height, and in post-menopausal or oophorectomized women, age. Weight and parity also had additional explanatory power for pre- and post-menopausal women. Those who were in the highest quintile of BMC but had never taken HRT showed significantly elevated percentage free oestradiol compared with those in the lowest quintile. Thus HRT is osteoprotective if taken after the menopause for more than 24 months.