The most physiologically active of the estrogens, estradiol is predominantly produced in the ovaries and metabolized in the liver. The synthesis of the sex hormone often involves the intermediate formation of testosterone, the primary male sex hormone, which is an indication of the close connection between the two steroids. The other main naturally occurring estrogens found in the bodies of humans and mammals are estrone and estriol, both of which can be formed from estradiol. Due to their function in female reproduction, estrogens, especially estradiol, are commonly utilized in oral contraceptives and other substances designed as methods of birth control, such as Depo-Provera.