Steroids are 21-carbon, 4-ring molecules. Biologically active steroids have a double covalent bond between carbon atoms 4 and 5, and a ketone groups at C3. Substitutions to the molecule can change the nature of a steroid's activity.
Endogenous corticosteroids are essential for life; they regulate a variety of functions under normal physiological conditions and also have effects when the body is under stress. All corticosteroids are produced in the adrenal cortex, but different sub-categories are synthesised in different areas of this. Mineralocorticoids, such as aldosterone, are produced in the zona glomerulosa. The glucocorticoids include cortisol (or hydrocortisone), which is produced in the zona fasiculata, and corticosterone from the zona reticularis.