Fig. 3. Major biosynthetic pathways in steroidogenesis.

The precursor of all steroid hormones is cholesterol. Steroidogenesis (synthesis of steroid hormones) occurs predominantly in the adrenal glands and gonads (ovaries and testes); however, it also occurs in the placenta. The key enzymes are located in the mitochondria and the endoplasmic reticulum of the cell. The first step in steroidogenesis involves the conversion of cholesterol to pregnenolone (a non-hormonal intermediate) within the cell mitochondria by the cholesterol side chain cleavage (CSCC) enzyme. The major classes of steroid hormones include progestogens, androgens, oestrogens and corticosteroids (glucocorticoids and mineralocorticoids). All steroid hormones have a cyclopentanoperhydrophenanthrene ring, and are further classified based on the number of carbon atoms. Pregnanes have 21 carbon atoms and are called C21 steroids (progestogens and corticosteroids). Oestranes have 18 carbon atoms (C18 steroids) and include oestrogens. Androstanes have 19 carbon atoms (C19 steroids) and include androgens. OH, hydrolase; HSD, hydroxysteroid dehydrogenase; DHT, dihydrotestosterone; DHEA, dehydroepiandrosterone; DHEAS, dehydroepiandrosterone sulfate.