Detailed definition
Estriol, abbreviated E3, is the third human estrogen. Outside of pregnancy, serum estriol is barely measurable; during pregnancy, the fetoplacental unit produces large amounts and serum levels can rise to nanomolar concentrations. Estriol binds estrogen receptors with substantially lower potency than estradiol, particularly at ERα, and its short biological half-life means it produces a less sustained estrogenic stimulus per dose than estradiol. The FDA has not approved any estriol-only or estriol-containing systemic product for menopause in the United States, although estriol is widely available abroad — in particular in compounded and pharmacy products in parts of Europe where it has been used for decades for genitourinary symptoms. In the US it appears in some compounded preparations marketed as "Bi-est" (typically a fixed ratio of estradiol to estriol) or "Tri-est" (estradiol, estriol, estrone). Major menopause societies do not endorse Bi-est or Tri-est over single-molecule estradiol.
Why it matters in menopause
Estriol shows up frequently in marketing of compounded HRT, often framed as the "safer" or "more natural" estrogen. The available evidence does not show that adding estriol improves outcomes over estradiol alone, and standardized dosing across compounded estriol products is not guaranteed. ClearedRx clinicians prescribe estradiol as the standard estrogen and only consider estriol-containing compounds when there is a specific patient-level reason.
Related terms
Sources
External references: Wikipedia.