Congenital adrenal hyperplasia (CAH) is the commonest inherited (congenital) endocrine (hormone) disorder caused by mutations in genes coding for enzymes in the adrenal gland that synthesis steroid hormones. Patients with CAH have adrenal insufficiency, that is a deficiency in the stress hormone cortisol. Their body’s response to this is to try and push the adrenal glands harder and they therefore enlarge in size (hyperplasia).

CAH results from mutations in the genes that code for enzymes that make steroid hormones in the adrenal gland. The most common enzyme affected is 21-hydroxylase and this enzyme is essential for the generation of cortisol and aldosterone. Depending on the severity of the enzyme deficiency, patients with CAH may either have just a deficiency of cortisol or a combined deficiency of cortisol and aldosterone.

Patients with CAH essentially have two problems; a deficiency of cortisol with or without aldosterone deficiency and an excess of adrenal androgens such as testosterone. The excess of male hormone such as testosterone leads to many of the problems experienced by patients with CAH such as virilisation of the female infant, early (precocious) puberty in both boys and girls and infertility problems in men and women. Throughout life patients with CAH are prone to adrenal crisis due to their cortisol deficiency.