Steroids



A steroid is a lipid characterized by a carbon skeleton with four fused rings. All steroids are derived from the acetyl CoA biosynthetic pathway. Different steroids vary in the functional groups attached to these rings. Hundreds of distinct steroids have been identified in plants, animals, and fungi.

Physiology and medicine
Their most important role in most living systems is as hormones. Steroid hormones produce 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.

In human physiology and medicine, the most important steroids are cholesterol, the steroid hormones, and their precursors and metabolites. In the bloodstream steroids are bound to carrier proteins.

Cholesterol is an important steroid alcohol, being a common component of animal cell membranes. However, a high level of it can cause various conditions and diseases, such as atherosclerosis. Most other steroids are synthesized from cholesterol. Also, various hormones, including vertebrate sex hormones, are steroids created from cholesterol.

Functional
Some of the common categories of steroids include:
 * Anabolic steroids are a class of steroids that interact with androgen receptors to increase muscle and bone synthesis. There are natural and synthetic anabolic steroids. These are the "steroids" used by athletes to increase performance.
 * Corticosteroids include glucocorticoid and mineralocorticoids:
 * Glucocorticoids regulate many aspects of metabolism and immune function, and are often prescribed by doctors to reduce inflammatory conditions like asthma and arthritis.
 * Mineralocorticoids are corticosteroids that help maintain blood volume and control renal excretion of electrolytes.
 * Sex steroids are a subset of sex hormones that produce sex differences or support reproduction. They include androgens, estrogens, and progestagens.
 * Phytosterols - steroids naturally occurring in plants.
 * Ergosterols - steroids occuring in fungi. This includes some Vitamin D supplements.

In Anatomical Therapeutic Chemical Classification System system, anabolic steroids are classified under A14, sex hormones under G03 and L02, and corticosteroids under A07EA, C05AA, D07, D10AA, H02, R01AD, R03BA, S01BA, S02B, and S03B.

Structural
It is also possible to classify steroids based upon their chemical composition. One example of how MeSH performs this classification is available at MeSH_D04.

Terminology
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.

Origin
Sex steroids include estrogen (U.S spelling) or oestrogen (UK spelling), progesterone and androgen. Oestrogen and progesterone are made primarily in the ovary and in the placenta during pregnancy and testosterone in the testis.