Bioavailability of oestriol

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Abstract
A rapid radioimmunoassay, using a specific antiserum against estriol (E3) was developed for the analyses of unconjugated estriol in post-menopausal women before and after oral administration of E3. Plasma [1 ml] was extracted once with 10 ml diethyl ether and assayed without further purification. The plasma levels of E3 in post-menopausal women appear to be below 5 pg/ml. When 10 ml plasma from a pool was assayed 2.9 pg/ml was found. When 1 ml was assayed from individual patients levels between 10-20 pg was found, which was not different from the plasma blank. A rise in plasma E3 was found already 15 min after oral administration of 6 mg or 12 mg E3 (Ovesterin). On the average there was a plasma E3 peak after 45 min and then a gradual decrease to low levels within 3 to 4 h. Evidently E3 is rapidly absorbed when administered orally. However, the duration of the plasma E3 elevation is short. E3 is rapidly metabolized. The short elevation probably explains the weak estrogenic effect exerted by E3 when administered once a day as receptor affinity for E3 is low.