Complex chemical diagram
Steroid ring system: The parent ABCD steroid ring system (hydrocarbon framework) is shown with IUPAC-approved ring lettering and atom numbering.[1]:1785f

A steroid is a biologically active organic compound with four rings arranged in a specific molecular configuration. Steroids have two principal biological functions: as important components of cell membranes which alter membrane fluidity; and as signaling molecules. Hundreds of steroids are found in plants, animals and fungi. All steroids are manufactured in cells from the sterols lanosterol (opisthokonts) or cycloartenol (plants). Lanosterol and cycloartenol are derived from the cyclization of the triterpene squalene.[2]

The steroid core structure is composed of seventeen carbon atoms, bonded in four "fused" rings: three six-member cyclohexane rings (rings A, B and C in the first illustration) and one five-member cyclopentane ring (the D ring). Steroids vary by the functional groups attached to this four-ring core and by the oxidation state of the rings. Sterols are forms of steroids with a hydroxy group at position three and a skeleton derived from cholestane.[1]:1785f[3] Steroids can also be more radically modified, such as by changes to the ring structure, for example, cutting one of the rings. Cutting Ring B produces secosteroids one of which is vitamin D3.

Examples include the lipid cholesterol, the sex hormones estradiol and testosterone,[4]:10–19 and the anti-inflammatory drug dexamethasone.[5]

Filled-in diagram of a steroid
Space-filling representation
Ball-and-stick diagram of the same steroid
Ball-and-stick representation
5α-dihydroprogesterone (5α-DHP), a steroid. The shape of the four rings of most steroids is illustrated (carbon atoms in black, oxygens in red and hydrogens in grey). The apolar "slab" of hydrocarbon in the middle (grey, black) and the polar groups at opposing ends (red) are common features of natural steroids. 5α-DHP is an endogenous steroid hormone and a biosynthetic intermediate.


Chemical diagram
A gonane (steroid nucleus)
Chemical diagram
Steroid 5α and 5β stereoisomers[1]:1786f

Gonane, also known as steran or cyclopentaperhydrophenanthrene, the simplest steroid and the nucleus of all steroids and sterols,[6][7] is composed of seventeen carbon atoms in carbon-carbon bonds forming four fused rings in a three-dimensional shape. The three cyclohexane rings (A, B, and C in the first illustration) form the skeleton of a perhydro derivative of phenanthrene. The D ring has a cyclopentane structure. When the two methyl groups and eight carbon side chains (at C-17, as shown for cholesterol) are present, the steroid is said to have a cholestane framework. The two common 5α and 5β stereoisomeric forms of steroids exist because of differences in the side of the largely planar ring system where the hydrogen (H) atom at carbon-5 is attached, which results in a change in steroid A-ring conformation. Isomerisation at the C-21 side chain produces a parallel series of compounds, referred to as isosteroids.[8]

Examples of steroid structures are:

In addition to the ring scissions (cleavages), expansions and contractions (cleavage and reclosing to a larger or smaller rings)—all variations in the carbon-carbon bond framework—steroids can also vary:

  • in the bond orders within the rings,
  • in the number of methyl groups attached to the ring (and, when present, on the prominent side chain at C17),
  • in the functional groups attached to the rings and side chain, and
  • in the configuration of groups attached to the rings and chain.[4]:2–9

For instance, sterols such as cholesterol and lanosterol have an hydroxyl group attached at position C-3, while testosterone and progesterone have a carbonyl (oxo substituent) at C-3; of these, lanosterol alone has two methyl groups at C-4 and cholesterol (with a C-5 to C-6 double bond) differs from testosterone and progesterone (which have a C-4 to C-5 double bond).

Chemical diagram
Cholesterol, a prototypical animal sterol. This structural lipid and key steroid biosynthetic precursor.[1]:1785f
Chemical diagram
5α-cholestane, a common steroid core
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