oestradiol

Oestradiol, or estradiol (E2), is the main estrogen hormone in humans. The most potent naturally occurring form is 17β-oestradiol. It is produced mainly by ovarian follicles in premenopausal individuals; during pregnancy the placenta contributes, and adipose tissue can produce it by aromatizing androgens. "Oestradiol" is the British spelling; "estradiol" is common in American usage.

Functions: Oestradiol drives development and maintenance of the female reproductive tract and secondary sexual characteristics, regulates the menstrual cycle, and supports bone density, lipid metabolism, and certain brain functions. It acts by binding estrogen receptors ERα and ERβ, with genomic and non-genomic signaling. Circulating levels and receptor distribution determine its tissue effects.

Clinical use: In menopause, estradiol provides symptom relief and reduces bone loss; if the uterus is intact, it is given with a progestin to lower endometrial cancer risk; otherwise estrogen alone is used. Estradiol is a component of many combined oral contraceptives and is used in transfeminine hormone therapy via oral, transdermal, or injectable forms.

Safety: Estrogen therapy carries risks of venous thromboembolism, stroke, myocardial infarction, and breast cancer with long-term use, influenced by age, smoking, dose, and route. Contraindications include a history of thromboembolic disease or estrogen-sensitive cancer. Drug interactions can affect estradiol metabolism via hepatic enzymes.

Pharmacokinetics: Orally administered estradiol undergoes extensive first-pass metabolism, limiting bioavailability; transdermal and other non-oral routes provide steadier systemic exposure. It is metabolized in the liver to conjugates and excreted in urine and bile. Common formulations include estradiol valerate as a prodrug and various patches, gels, and injections.