The male hormone testosterone moves about the body in a free state (about 2% in normal men) or bound to one of two plasma proteins: sex hormone-binding globulin (SHBG) or albumin. Only when testosterone is in its free state can it interact with cellular receptors (e.g., trigger muscle-building activity). Testosterone has two metabolites that pose potential problems for aging males and also for bodybuilders and athletes. One is estradiol (testosterone is converted to estradiol by the enzyme aromatase) and the other is dihydrotestosterone (DHT) (testosterone is converted to DHT by the enzyme 5-alpha reductase). As men age, there is a dramatic decrease in testosterone production, but without a corresponding decrease in the production of estradiol and DHT. Even though testosterone levels have plummeted, the aromatization of testosterone to estradiol is maintained, or even accelerated, and more testosterone is also being aromatized in fatty tissues. This creates excess levels of estradiol and, as a consequence, a blood level ratio of testosterone to estradiol that continues to decline, tipping the scale in balance of an estradiol-dominant environment.