Estradiol (estradiol-17 beta, E2) is part of an estrogen that is a group of steroids that regulate the menstrual cycle and function as the main female sex hormones. Estrogens are responsible for the development of female sex organs and secondary sex characteristics and are tied to the menstrual cycle and pregnancy. They are considered the main sex hormones in women and are present in small quantities in men. Estradiol (E2) is the predominant form of estrogen and is produced primarily in the ovaries with additional amounts produced by the adrenal glands in women and in the testes and adrenal glands in men. Estradiol levels are used in evaluating ovarian function. Estradiol levels are increased in cases of early (precocious) puberty in girls and gynecomastia in men. Its main use has been in the differential diagnosis of amenorrhea – for example, to determine whether the cause is menopause, pregnancy, or a medical problem. In assisted reproductive technology (ART), serial measurements are used to monitor follicle development in the ovary in the days prior to in vitro fertilization. Estradiol is also sometimes used to monitor menopausal hormone replacement therapy.