estrogen

Estrogen is the principal female sex hormone, the driver of the changes of puberty and the menstrual cycle in women, though both sexes make and need some. Beyond reproduction it quietly protects bone, shapes cholesterol levels, and affects the lining of the uterus. As medicine it is a key ingredient in birth control pills and in hormone therapy for menopause.

The main natural estrogen is estradiol, a steroid hormone produced chiefly by the ovaries. It enters cells and binds estrogen receptors, a kind of nuclear receptor, altering gene expression in responsive tissues including the uterus, breast, bone, and blood vessels. Therapeutic estrogens include estradiol itself and synthetic forms such as ethinylestradiol, used in oral contraceptives because it survives oral dosing better. They are available as pills, patches, gels, and vaginal preparations.

Clinically, estrogen is combined with a progestin in most contraceptive pills, where it helps suppress ovulation and stabilize the cycle, and is used in menopausal hormone replacement to relieve hot flushes and protect bone. The important caveat is risk: estrogen-containing products increase the chance of venous blood clots and, in certain settings, stroke and some cancers, and they are generally avoided in smokers over 35 and those with clotting disorders. Estrogen given without a progestin can overstimulate the uterine lining in women who still have a uterus.