estriol

Estriol is a natural estrogen, a steroid hormone in the estrogen family. It is the least potent of the major estrogens in humans, with weaker binding to estrogen receptors than estradiol and estrone.

During pregnancy, estriol is produced predominantly by the placenta from precursors supplied by the fetus and mother. The placenta uses aromatase to convert 16α-hydroxy-DHEA derivatives into estriol, which then enters maternal circulation and is excreted in urine largely as conjugates.

Biological function and activity are comparatively modest; estriol contributes to the hormonal milieu of pregnancy but has limited systemic estrogenic effects relative to estradiol. Its role may include supporting uterine and vaginal tissue function during pregnancy, though its overall activity is weaker than that of other estrogens.

Clinical significance: Estriol is a biomarker used in prenatal screening. Levels of unconjugated estriol in maternal serum or urine are part of screening panels, such as the triple or quadruple tests, to assess fetal well-being and risk of certain abnormalities when interpreted alongside other markers like hCG and AFP.

Medical use: Estriol has been used in hormone replacement therapy in some vaginal estrogen products due to lower systemic exposure, and is incorporated into certain regional formulations. Systemic estrogen therapy more commonly relies on other estrogens, and use should follow contemporary medical guidelines and regulatory approvals.

Chemistry: Estriol is a triphenolic steroid estrogen with three hydroxyl groups, distinguishing it from the more potent estrogens estradiol and estrone.