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Formation of Ethinyl Estradiol in Women during Treatment with Norethindrone Acetate

Columbia University College of Physicians and Surgeons (M.C.C., R.A.L.), New York, New York 10032; and University of Southern California School of Medicine, Women’s & Children’s Hospital (X.Z., E.G., F.Z.S.), Los Angeles, California 90033

Address all correspondence and requests for reprints to: Rogerio A. Lobo, M.D., 622 West 168th Street, PH 16-69, New York, New York 10032. E-mail: ral35{at}columbia.edu.

Context: The significance of the conversion of norethindrone acetate (NET-A), a commonly prescribed, synthetic, oral progestogen, to ethinyl estradiol (EE₂) is controversial.

Objective: We sought to determine accurately the extent to which NET-A may be converted to EE₂, and if so, whether circulating levels of EE₂ would be of clinical significance.

Design, Subjects, and Interventions: We administered NET-A 10, 20, or 40 mg once daily for 7 d to 20 regularly menstruating premenopausal women and measured NET-A and EE₂ levels before drug intake; at 1, 2, 4, 8, and 24 h after the first dose; and 2 h after the seventh dose.

Results: The mean EE₂ maximum serum concentration obtained from the 10-, 20-, and 40-mg doses of NET-A in this study is 58, 178, and 231 pg/ml, respectively. The conversion ratio of NET-A to EE₂ ranged from 0.20 to 0.33% for the different doses.

Conclusions: NET-A is converted to EE₂, and although the conversion rate is relatively small, higher doses of NET-A, as used clinically, give rise to substantial levels of EE₂.

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