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Estrogens are hormones that are important for sexual and reproductive development, mainly in women. They are also referred to as female sex hormones. The term "estrogen" refers to all of the chemically similar hormones in this group, which are estrone, estradiol (primary in women of reproductive age) and estriol.
In women, estrogen is produced mainly in the ovaries, but it is also produced by fat cells and the adrenal gland. Estrogen is involved in the onset of puberty, playing a role in the development of so-called secondary sex characteristics, such as breasts, and pubic and armpit hair.
Estrogen also helps regulate the menstrual cycle, controlling the growth of the uterine lining during the first part of the cycle. If the woman's egg is not fertilized, estrogen levels decrease sharply and menstruation begins. If the egg is fertilized, estrogen works with progesterone, another hormone, to stop ovulation during pregnancy.
Estrogen controls lactation and other changes in the breasts, including at adolescence and during pregnancy.
During pregnancy, the placenta produces estrogen, specifically the hormone estriol.
Estrogen is instrumental in bone formation, working with vitamin D, calcium and other hormones to effectively break down and rebuild bones according to the body's natural processes. As estrogen levels start to decline in middle age, the process of rebuilding bones slows, with postmenopausal women eventually breaking down more bone than they produce. This is why postmenopausal women are four times more likely to suffer from osteoporosis than men, according to the Cleveland Clinic.
Estrogen also plays a role in blood clotting, maintaining the strength and thickness of the vaginal wall and the urethral lining, vaginal lubrication and a host of other bodily functions. It affects skin, hair, mucous membranes and the pelvic muscles, according to Johns Hopkins Medicine. The hormone also affects the brain, and studies also show that chronically low estrogen levels are linked with a reduced mood.
Men produce estrogen as well, but at lower levels than women. In men, estrogen is thought to affect sperm count.
Changes in estrogen levels
Estrogen levels naturally increase during puberty, and also during pregnancy.
Estrogen levels fall after menopause, or when a woman stops menstruating. This reduction in estrogen production can cause symptoms such as hot flashes, vaginal dryness and loss of sex drive.
Other conditions that can cause estrogen levels to drop include hypogonadism (or diminished function of the ovaries) and polycystic ovarian syndrome. Extreme exercise and anorexia can also cause a decrease in estrogen levels because women with low body fat may not be able to produce adequate amounts of estrogen.
Estrogen levels also decrease after childbirth.
Medications with estrogen
Estrogen is found in most oral birth control pills (along with the hormone progestin.) Estrogen helps stop ovulation during pregnancy, and birth control pills mimic this effect by regulating the levels of estrogen and thereby preventing ovulation from occurring.
Hormone replacement therapy — a treatment to reduce symptoms of menopause — also includes estrogen (which can be given in combination with progestin). For many years, the therapy was used to treat postmenopausal problems such as hot flashes and vaginal atrophy — thinning, drying and inflammation of the vaginal walls due to a decrease in estrogen. But in 2002, a study by the Women's Health Initiative showed that hormone replacement therapy — both solely estrogen and estrogen-and-progestin — had significant risks. It increased the risk of breast cancer, stroke and blood clots, and did not, as predicted, lower the risk of heart disease. Given the danger, hormone replacement therapy should be prescribed on a case-by-case basis. It is currently approved for postmenopausal symptoms, though women who do start hormone replacement therapy are encouraged to try the smallest dose for the shortest amount of time, according to the Food and Drug Administration. Only women with a severe risk of osteoporosis who cannot take non-estrogen therapies should be considered for using hormone replacement therapy preventatively.
Estrogen's role in cancer
The majority of breast cancers are sensitive to estrogen, meaning that estrogen promotes tumor growth. These cancers are called hormone receptor positive breast cancers. For people with these cancers, treatments to lower estrogen levels or block estrogen production can be used to help prevent cancer recurrence after surgery, or to slow cancer growth.
Jessie Szalay contributed to this article.