Measuring the circulating levels of estradiol is important for assessing the ovarian function and monitoring follicular development for assisted reproduction protocols. Estradiol plays an essential role throughout the human menstrual cycle. Elevated estradiol levels in females may also result from primary or secondary ovarian hyperfunction. Very high estradiol levels are found during the induction of ovulation for assisted reproduction therapy or in pregnancy. Decreased estradiol levels in females may result from either lack of ovarian synthesis (primary ovarian hypofunction and menopause) or a lesion in the hypothalamus-pituitary axis (secondary ovarian hypofunction). Elevated estradiol levels in males may be due to increased aromatization of androgens, resulting in gynecomastia.