Anabolic-androgenic steroids represent an entirely different class of hormones. The prototype hormone of this class is the male hormone testosterone, which is secreted primarily by the testes in males. Anabolic-androgenic steroids produce masculinizing (androgenic) effects — such as beard growth, male pattern baldness, and male sexual characteristics — together with muscle-building (anabolic) effects. These latter effects account for the greater muscle mass and lower body fat of men as compared to women. Many synthetic anabolic-androgenic steroids have been developed over the last fifty years. Like testosterone, these synthetic substances produce both anabolic and androgenic effects; there are no purely anabolic or purely androgenic compounds. In medical practice, the principal use of testosterone is in the treatment of hypogonadal men — men who do not secrete sufficient testosterone in their own bodies, and who therefore require testosterone supplementation to maintain normal masculine characteristics. Aside from this application, anabolic-androgenic steroids have only very limited medical uses, such as in the treatment of certain forms of anemia.