Norethisterone is a potent progestogen and binds to the PR with approximately 150% of the affinity of progesterone. In contrast, its parent compounds, testosterone, nandrolone (19-nortestosterone), and ethisterone (17α-ethynyltestosterone), have 2%, 22%, and 44% of the relative binding affinity of progesterone for the PR. Unlike norethisterone, its major active metabolite 5α-dihydronorethisterone (5α-DHNET), which is formed by transformation via 5α-reductase, has been found to possess both progestogenic and marked antiprogestogenic activity, although its affinity for the PR is greatly reduced relative to norethisterone at only 25% of that of progesterone. Norethisterone produces similar changes in the endometrium and vagina, such as endometrial transformation, and is similarly antigonadotropic, ovulation-inhibiting, and thermogenic in women compared to progesterone, which is in accordance with its progestogenic activity.