Any of a class of natural or synthetic organic compounds with a molecular core, or nucleus, of 17 carbon atoms in a characteristic three-dimensional arrangement of four rings. The configuration of the nucleus, the nature of the groups attached to it, and their positions distinguish different steroids. Hundreds have been found in plants and animals and thousands more synthesized or made by modifying natural steroids. Steroids are important in biology, chemistry, and medicine. Examples include many hormones (including the sex hormones), bile acids, sterols (including cholesterol), and oral contraceptives (see contraception). Digitalis was the first steroid widely used in Western medicine. Corticosteroids (see cortisone) and their synthetic analogs are used to treat rheumatism and other inflammatory ailments. Seealso anabolic steroid.
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Steroids vary by the functional groups attached to these rings and the oxidation state of the rings. Hundreds of distinct steroids are found in plants, animals, and fungi. All steroids are made in cells either from the sterol lanosterol (animals and fungi) or the sterol cycloartenol (plants). Both sterols are derived from the cyclization of the triterpene squalene.
Steroids include estrogen (US spelling) or oestrogen (UK/AUS spelling), progesterone and testosterone. Estrogen and progesterone are made primarily in the ovary and in the placenta during pregnancy and testosterone in the testes. Testosterone is also converted into estrogen to regulate the supply of each, in the bodies of both females and males. Certain neurons and glia in the central nervous system (CNS) express the enzymes that are required for the local synthesis of pregnane neurosteroids, either de novo or from peripherally derived sources. The rate limiting step of steroid synthesis is the conversion of cholesterol to pregnenolone which occurs inside the mitochondrion.
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