Detailed definition
The liver is the primary site of drug metabolism for most oral medications. Phase I reactions (oxidation, reduction, hydrolysis) are largely catalyzed by the cytochrome P450 enzyme family, particularly CYP3A4, CYP2D6, CYP2C9, and others. Phase II reactions (glucuronidation, sulfation, methylation, glutathione conjugation) make metabolites more water-soluble for excretion. Estrogens undergo extensive hepatic metabolism: estradiol is rapidly oxidized to estrone (CYP3A4, CYP1A2) and conjugated to inactive forms. Oral estrogen also stimulates hepatic synthesis of plasma proteins (SHBG, TBG, ceruloplasmin, certain clotting factors), which is the mechanism behind the modestly elevated VTE risk with oral estrogen. Drug-drug interactions through CYP3A4 (e.g., with rifampin, carbamazepine, St. John's wort) can affect estrogen levels.
Why it matters in menopause
Hepatic metabolism explains both why oral estradiol is much less bioavailable than transdermal and why oral estrogen has a different safety profile (VTE, lipid changes). Drug interaction screening is part of safe HRT prescribing.
Related terms
Sources
External references: Wikipedia.