Radioimmunoassay of plasma norethisterone after ethynodiol diacetate administration

A radioimmunoassay for norethisterone was developed and used to measure plasma levels of the compound in women following oral administration of tablets containing 1 mg ethynodiol diacetate and 0.05 mg ethinyloestradiol. The results showed that the norethisterone assay was adequately specific, sensitive and accurate for application to studies of the absorption and clinical pharmacology of orally administered ethynodiol diacetate. Peak concentrations of norethisterone (between 7.22 and 15.35 ng/ml) were reached between 1 1 2 and 3 h after dosage and norethisterone could be measured in plasma up to 48 h after a single oral dose of ethynodiol diacetate. Comparison of mean area under the concentration-time curve (AUC) data indicated that measurement of plasma norethisterone could be used to monitor the absorption of orally administered ethynodiol diacetate.