Estradiol (17β-estradiol) (also oestradiol) is a sex hormone. Labelled the "female" hormone but also present in males, it represents the major estrogen in humans. Estradiol has not only a critical impact on reproductive and sexual functioning, but also affects other organs including bone structure. Estradiol is the most commonly measured type of estrogen for nonpregnant women. The amount of estradiol in a woman's blood varies throughout her menstrual cycle. After menopause, estradiol production drops to a very low but constant level. In women, estrogens are produced mainly in the ovaries. Small amounts are also produced by the adrenal glands. In men, the adrenal glands and testicles produce small amounts of estrogens.
Small amounts of estrone are made throughout the body in most tissues, especially fat and muscle. This is the major source of estrogen in women who have gone through menopause. Estradiol is the primary reproductive hormone in nonpregnant women. Estradiol influences the maturation and maintenance of the uterus during the normal menstrual cycle. After ovulation, estradiol levels initially fall abruptly, but then increase. At the end of the cycle, levels fall off in anticipation of the initiation of the next cycle. Estradiol levels are generally low in menopause due to diminished ovarian production.
The effect of estradiol (and estrogens) upon male reproduction is complex. Estradiol is produced in the Sertoli cell of the testes. There is evidence that estradiol is to prevent apoptosis of male germ cells. The male testes produce a small amount of estradiol. Elevated levels in males can lead to gynocomastia (breast tenderness or soreness). Increased body fat that can deactivate male androgens may cause increased estradiol levels in males. Levels in men can also be increased by excessive use of marijuana, alcohol, or prescribed drugs, including phenothiazines and spironolactone. Estradiol levels can also be dramatically elevated in testicular or ovarian tumors and tumors of a number of glands in both men and women. Males with sex chromosome genetic conditions such as Klinefelters Syndrome will have a higher level of estradiol.