Östriol

Östriol, known in English as estriol, is a naturally occurring estrogen and is considered the weakest of the three major estrogens in humans, along with estradiol (E2) and estrone (E1). It is produced in small amounts in nonpregnant individuals but becomes the predominant estrogen in the placenta during pregnancy.

Biosynthesis and distribution

In nonpregnant women, circulating estriol levels are low. During pregnancy, the placenta converts precursors supplied by both the fetal- and maternal-fetal unit, notably from dehydroepiandrosterone (DHEA) derivatives produced by the fetal adrenal glands and maternal tissues, into estriol. This placental production leads to a marked rise in estriol levels in the second and third trimesters. Estriol can be measured in blood or urine and has historically been used as part of pregnancy screening tests.

Biological activity

Estriol has estrogenic effects by binding to estrogen receptors, but with much lower affinity and potency than estradiol. Because of its weaker activity, estriol is often described as having a milder estrogenic effect on tissues such as the endometrium and vaginal epithelium. In pregnancy, estriol contributes to several estrogenic actions of the placenta but is not the primary driver of early reproductive function.

Clinical use and regulation

Estriol is used in some hormone replacement therapy regimens, particularly in certain regions, usually in combination with other estrogens or progestins. Its relatively low potency has historically led to claims of a different safety profile, but evidence remains mixed, and risks associated with estrogen therapy (such as thromboembolism and breast cancer) are still considered. In obstetrics, estriol has been employed as part of pregnancy screening panels (historically in the triple screen), though modern practice relies on a broader set of markers.

See also: estradiol, estrone, estrogens.