Pharmacokinetics of Estrogens

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Chapter: Essential pharmacology : Estrogens, Progestins and Contraceptives

Estrogens are well absorbed orally and transdermally, but natural estrogens are inactive by the oral route due to rapid metabolism in liver. Estradiol esters injected i.m. are slowly absorbed and exert prolonged action.


PHARMACOKINETICS

 

Estrogens are well absorbed orally and transdermally, but natural estrogens are inactive by the oral route due to rapid metabolism in liver. Estradiol esters injected i.m. are slowly absorbed and exert prolonged action. Natural estrogens in circulation are largely plasma protein bound—to SHBG as well as to albumin.

 

Estradiol is converted to estrone and vice versa in liver. Estriol is derived from estrone. All three are conjugated with glucuronic acid and sulfate— excreted in urine and bile. Considerable enterohepatic circulation occurs due to deconjugation in intestines and reabsorption—ultimate disposal occurs mostly in urine.

 

Ethinylestradiol is metabolized very slowly (t½ 12–24 hours)—orally active and more potent.

 

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