A steroid is a type of organic compound that contains a characteristic arrangement of four cycloalkane rings joined to one another. Examples of steroids include the dietary lipid cholesterol, the sex hormones estradiol and testosterone,[1]:10–19 bile acids, and drugs such as the anti-inflammatory agent dexamethasone.[2]

The steroid core is composed of seventeen carbon atoms bonded together in the form of four fused rings: three cyclohexane rings (designated as rings A, B and C in the figure to the right) and one cyclopentane ring (the D ring). Individual steroids vary, first and primarily, by the oxidation state of the carbon atoms in the rings and by the chains and functional groups attached to this four-ring system; second, steroids can vary more markedly via changes to the ring structure (e.g., via ring scissions that produce secosteroids like vitamin D3, see below). Sterols are a particularly important form of steroids, with sterols having a cholestane-derived framework and an hydroxyl group at the C-3 ring position being the most prominent (e.g., as in cholesterol, shown at right).[3]

Hundreds of distinct steroids are found in animals, fungi, plants, and elsewhere. All natural steroids are made in living cells, either from the sterol lanosterol (animals and fungi, see examples) or from cycloartenol (plants). Both lanosterol and cycloartenol are derived via cyclization of the triterpenoid squalene.[4]

The parent ABCD steroid ring system, and IUPAC-approved ring lettering and atom numbering.[3] The 17-carbon compound gonane, a hydrocarbon composed only of the ABCD-ring (without branches or substitution), is the simplest steroid and a substructure present in most steroids.

5α-Cholestane, a 27 carbon steroid hydrocarbon with ABCD-ring system, 2 axial methyl groups attached at C-10 and C-13, and an 8 carbon side chain at C-17—the carbon framework present in cholesterol and many other steroids.

Cholesterol, a prototypical animal sterol, and precursor via steroidogenesis of most other animal steroids; shown with its approved ring and atom conventions.[3]
Contents [hide]
1 Nomenclature and examples
2 Types
2.1 Taxonomic/functional
2.2 Structural: Intact ring system
2.3 Structural: Cleaved, contracted, and expanded rings (seco-, nor-, and homosteroids)
3 Biological significance
4 Pharmacological actions
5 Biosynthesis and metabolism
5.1 Mevalonate pathway
5.2 Steroidogenesis
5.3 Regulation
5.4 Alternative pathways
6 Metabolism
7 Isolation, structure determination, and methods of analysis
8 Chemical synthesis of steroids
8.1 Microbial transformations
8.2 Partial and total chemical synthesis
9 History
10 See also
11 References
12 Further reading
13 External links
Nomenclature and examples[edit]
As IUPAC guidance notes (and is explained more fully following the quote),

"Steroids are compounds possessing the skeleton of cyclopenta[a]phenanthrene or a skeleton derived therefrom by one or more bond scissions or ring expansions or contractions. Methyl groups are normally present at C-10 and C-13. An alkyl side chain may also be present at C-17. Sterols are steroids carrying a hydroxyl group at C-3 and most of the skeleton of cholestane. Additional carbon atoms may be present in the side chain."[5]
Gonane (see opening image, above) is the simplest possible steroid and is composed of seventeen carbon atoms in carbon-carbon bonds that form four fused rings in a defined three-dimensional shape. The three cyclohexane rings (designated as rings A, B, and C in the figures above form the skeleton of a perhydro- derivative of phenanthrene. The D-ring has a cyclopentane structure; hence, though it is uncommon, per IUPAC steroids can also be named as various hydro-derivatives of cyclopenta[a]phenanthrene.[6] When the two methyl groups and 8 carbon side chain (at C-17, as shown for cholesterol) are present, the steroid is said to have a cholestane framework. The following are some examples of