Steroids are 21-carbon, 4-ring molecules, with biologically active steroids have a double covalent bond between carbon atoms 4 and 5, and a ketone groups at C3. The body endogenously produces steroids which are essential for life; they regulate a variety of functions under normal physiological conditions and have important roles in response to stress. These steroids are produced in the adrenal cortex and are therefore known as "corticosteroids". The corticosteroids can be further divided to mineralocorticoids and glucocorticoids which are synthesised in different areas of the cortex. Mineralocorticoids (such as aldosterone) are produced in the zona glomerulosa; glucocorticoids include cortisol (from the zona fasiculata) and corticosterone (from the zona reticularis). There is some overlap in function of mineralo- and gluco-corticoids.
Corticosteroids are synthesised from plasma cholesterol. This is stored in the adrenal gland and corticosteroids are assimilated and released as they are required.