Endogenous estradiol levels above 10 pg/mL and higher levels of endogenous testosterone decrease the bone remodeling rate and protect bone mass in women 65 years of age or younger. The new estradiol data are much lower than the 40 to 60 pg/mL that has been cited in the older literature. In this population, estradiol levels higher than 10 pg/mL were associated with a higher bone mineral density (BMD) of the lumbar spine (+14%, P