Estradiol is a form of estrogen, necessary for many processes in the body. Estradiol is the primary reproductive hormone in nonpregnant women that is involved in the development and maintenance of the female reproductive system. A small amount is produced by the male testes. Estradiol levels are used to help evaluate ovarian function and gynecomastia in men. Gynecomastia is swelling of the breast tissue in boys or men, caused by an imbalance of the hormones estrogen and testosterone.