While estradiol and estrone are the two most common estrogens discussed when referring to women's hormones, estriol is a third, very important hormone females produce. This is produced in high amounts during pregnancy by the placenta. When not pregnant, women produce small amounts of estriol however.

Considered the weakest of all of the estrogens, with just one-seventh the strength of estradiol[4], estriol has shown promise as an alternative therapy with many applications. Estriol may be used to treat many hormone related issues including vaginal dryness and urinary frequency in menopause. It may also be used for anti-aging purposes and facial wrinkles.

Estriol can be a weak or strong estrogen depending on if it is given acutely or chronically when given to immature animals, but is an antagonist when given in combination with estradiol. Estriol may play a role in the development of breast cancer, but based on in vitro research, does appear to act as an antagonist to the G-protein coupled estrogen receptor.

Though estriol is used as part of the primarily North American phenomenon of bioidentical hormone replacement therapy, it is not approved for use by the FDAor Health Canada. Though initial research in the 1970s suggested it could be used therapeutically as an estrogen, subsequent research failed to confirm this hypothesis.