One aim of this study is to determine changes in body composition and hormones that differentiate athletes who stop getting their periods versus those who continue to get their periods and non-athletes. The second aim of this study is to determine whether transdermal or oral estrogen (versus no estrogen) is effective in increasing bone density and improving bone microarchitecture in adolescent athletes who are not getting their periods and are thus estrogen deficient. The investigators hypothesize that transdermal estrogen will be more effective than oral estrogen or no estrogen in improving bone health in amenorrheic adolescent athletes.