Estradiol is a sex hormone that is present in both males and females, and is the most important form of estrogen in humans. In women, it plays a key role in the development and functioning of the reproductive system, as well as in the growth of certain bones. It also governs the distribution of body fat in women and is the main hormone responsible for the fact that women are shorter than men, on average. Both the ovaries and the adrenal glands produce it.
The menstrual cycle in women involves predictable variations in the levels of several hormones, with estradiol being one of them. It is involved in the ovulation process and prepares the inner lining of the uterus for implantation, should the egg be fertilized. Certain tests done on baboons and other primates have indicated that his hormone also plays a continual role in maintaining a pregnancy, helping it to last for the full gestation period.
As a sex hormone, it also triggers many of the developments of the reproductive system that begin in puberty. It is present throughout the reproductive years, and declines during and after menopause. This decline is precisely what causes many of the symptoms of menopause, such as hot flashes and night sweats, vaginal dryness, and the loss of bone mass that may lead to osteoporosis.
Injections of estradiol are sometimes given to treat symptoms like these. They can also be used to compensate for ovary failure or similar conditions in women, and some forms of it can treat symptoms caused by prostate cancer in men. When given to women, it is necessary for them to use an effective form of birth control, due to the high risk of complications and birth defects that would exist in the event of a pregnancy.
Despite a common belief to the contrary, estrogens are important for male reproductive health and development as well. Estradiol is produced by a certain type of cell in the testes, and evidence suggests that it may be responsible for preventing the spontaneous death of sperm cells. Elevated levels can have negative effects, including a lowered sperm count. Males who have genetic conditions involving the sex chromosomes, such as Klinefelter's Syndrome, often have higher-than-normal levels. While it is primarily a sex hormone, estradiol also has complex interactions with the liver, governing the production of certain proteins, including those responsible for blood clotting.