A steroid is a type of lipid, characterized by a carbon skeleton with four fused rings. Different steroids vary in the functional groups attached to these rings. Hundreds of distinct steroids have been identified in plants and animals. Their most important role in most living systems is as hormones.
In human physiology and medicine, the most important steroids are cholesterol, the steroid hormones, and their precursors and metabolites.
In conversations, the term steroid is often used more restrictively in specific contexts. For instance, steroid in common medical usage by non-endocrinologists usually refers to corticosteroids, nearly always glucocorticoids. In an athletic or body-building context, steroid commonly refers to anabolic steroids.
Cholesterol is an important steroid, being a common component of animal cell membranes. However, a high level of it can cause various conditions and diseases, including atherosclerosis. Most other steroids are synthesized from cholesterol. Also, various hormones, including vertebrate sex hormones, are steroids created from cholesterol.
There are also various specific categories of steroids, including:
Steroid hormones have their physiological effects by binding to steroid hormone receptor proteins. The binding of steroid hormones to their receptors causes changes in gene transcription and cell function.
All steroid hormones are synthesized from pregnenolone, a derivative of cholesterol. Steroid hormones contain 21 or fewer carbon atom. The first step in steroid hormone synthesis is the removal of the 6 carbon side-chain of cholesterol. The enzyme desmolase cleaves hydroxylated bonds at C-20 and C-22. Three molecules of O2 and 3 molecules of NADPH are consumed in the conversion from cholesterol to pregnenolone. Adrenocorticotropic hormone (ACTH) stimulates this conversion.