noun, plural: estriols
An estrogenic hormone with a chemical formula of C18H24O3
Estriol (E3) is one of the major endogenous estrogens of humans and other vertebrates. Other endogenous estrogens are estrone (E1), estradiol (E2), and estetrol (E4). Estriol is an estrogenic hormone with a chemical formula of C18H24O3.
Estriol is a sex steroid hormone. It is one of the three major endogenous estrogens (others are estradiol and estrone). However, compared with the other major endogenous estrogens, the estriol is a weak estrogen. It has less affinity for estrogen receptors (ERα and ERβ). Unlike estrone, estriol cannot be converted into estradiol.
Unlike the estradiol and estrone, the estriol is not synthesized in the ovary. The estriol is produced chiefly in the liver. One of the main pathways of producing estriol is by the 16α-hydroxylation of estradiol and estrone through the action of cytochrome P450 enzymes (e.g. CYP3A4). The estriols released into the bloodstream are in very small quantities. In females that are not pregnant, the quantity of circulating estriols is very low. Nonetheless, the estriol levels in pregnant women significantly increase, about a thousand-fold. During pregnancy, the estriol produced in high amounts by the placenta and becomes the predominant form of estrogen.
Estriol is also produced synthetically for use as a medication for females needing menopausal hormone therapy.