Estradiol is the strongest of the three estrogen hormones. It is naturally produced in both genders, with much higher levels in females. It is predominantly produced within the ovarian follicles, but also in other tissues, such as the adrenal glands, fat, liver, breasts, brain, testes, and placenta (during pregnancy).
The primary function of estradiol in females is to mature and maintain the reproductive system, including the mammary glands, uterus, and vagina. It is responsible for the development of female secondary sexual characteristics (e.g. breasts, female fat distribution). During each menstrual cycle, estradiol levels increase to trigger the maturation and release of the egg, and the thickening of the uterus lining to allow a fertilized egg to implant. Estradiol also plays important functions in the male reproductive system, skeletal system, skin health, nervous system, and cardiovascular system.