It is specifically a derivative of testosterone in which an ethynyl group has been added at the C17α position and the methyl group at the C19 position has been removed; hence, it is a combined derivative of ethisterone (17α-ethynyltestosterone) and nandrolone (19-nortestosterone). These modifications result in increased progestogenic activity and oral bioavailability as well as decreased androgenic/anabolic activity. Derivatives
Norethisterone (NET) is the parent compound of a large group of progestins that includes most of the progestins known as the 19-nortestosterone derivatives. This group is divided by chemical structure into the estranes (derivatives of norethisterone) and the gonanes (18-methylgonanes or 13β-ethylestranes; derivatives of levonorgestrel) and includes the following marketed medications:

Several of these act as prodrugs of norethisterone, including NETA, NETE, etynodiol diacetate, lynestrenol, and quingestanol acetate. Noretynodrel may also be a prodrug of norethisterone. NETA is taken by mouth similarly to norethisterone, while NETE is given by injection into muscle. Non-17α-ethynylated
19-Nortestosterone (19-NT) progestins which are technically not derivatives of norethisterone (as they do not have a C17α ethynyl group) but are still closely related (with other substitutions at the C17α and/or C16β positions) include the following marketed medications:
The C17α vinyl (ethenyl) derivatives norgesterone (17α-vinyl-δ5(10)-19-NT) and norvinisterone (17α-vinyl-19-NT)
The C17α allyl derivatives allylestrenol (3-deketo-17α-allyl-19-NT) and altrenogest (17α-allyl-δ9,11-19-NT)
The C17α alkyl derivative normethandrone (17α-methyl-19-NT)
The C17α cyanomethyl derivative dienogest (17α-cyanomethyl-δ9-19-NT)
The C16β ethyl derivative oxendolone (16β-ethyl-19-NT)Many anabolic steroids of the 19-nortestosterone family, like norethandrolone and ethylestrenol, are also potent progestogens, but were never marketed as such. Synthesis
Chemical syntheses of norethisterone have been published.
These transformations are catalyzed by 5α- and 5β-reductase and 3α- and 3β-hydroxysteroid dehydrogenase both in the liver and in extrahepatic tissues such as the pituitary gland, uterus, prostate gland, vagina, and breast. With the exception of 3α,5α- and 3β,5α-tetrahydronorethisterone, which have significant affinity for the ER and are estrogenic to some degree, the 3,5-tetrahydro metabolites of norethisterone are inactive in terms of affinity for sex steroid receptors (specifically, the PR, AR, and ER). A small amount of norethisterone is also converted by aromatase into EE. Norethisterone is metabolized in the liver via hydroxylation as well, mainly by CYP3A4. Some conjugation (including glucuronidation and sulfation) of norethisterone and its metabolites occurs in spite of steric hindrance by the ethynyl group at C17α. The ethynyl group of norethisterone is preserved in approximately 90% of all of its metabolites.Norethisterone is used in birth control pills, opposed to progesterone itself, because it is not metabolized as rapidly as progesterone when consumed orally. When progesterone is consumed orally it is rapidly metabolized in the gastrointestinal tract and the liver, and broken down into many different metabolites. Whereas, norethisterone is not as rapidly metabolized allowing norethisterone to be present in higher quantities allowing it to more effectively compete for progesterone receptor binding sites. Elimination
Norethisterone is eliminated 33 to 81% in urine and 35 to 43% in feces. Chemistry
Norethisterone, also known as 17α-ethynyl-19-nortestosterone or as 17α-ethynylestra-4-en-17β-ol-3-one, is a synthetic estrane steroid and a derivative of testosterone.