Norsteroids (nor-, L. norma, from "normal" in chemistry, indicating carbon removal) are a structural class of steroids that have had an atom or atoms (typically carbon) removed, biosynthetically or synthetically, from positions of branching off of rings or side chains (e.g., removal of methyl groups), or from within rings of the steroid ring system. For instance, 19-norsteroids (e.g., 19-norprogesterone) constitute an important class of natural and synthetic steroids derived by removal of the methyl group of the natural productprogesterone; the equivalent change between testosterone and 19-nortestosterone (nandrolone) is illustrated below.
^International Union of Pure and Applied Chemistry (IUPAC), 1999, "RF-4.1 Removal of Skeletal Atoms," in "RF-4. Skeletal Modifications" in Revised Section F: Natural Products and Related Compounds (IUPAC Recommendations 1999), see , accessed 20 May 2014. See also IUPAC, 1976, "Nomenclature of Organic Chemistry: Section F - Natural Products and Related Compounds, Recommendations 1976", IUPAC Information Bulletin Appendices on Tentative Nomenclature, Symbols, Units, and Standards, No. 53, December, 1976, also in Eur. J. Biochem. 1978, 86, 1-8.