21-hydroxylase deficiency is the commonest cause of congenital adrenal hyperplasia. This leads to excess androgen production due to shunting of 17 OH progesterone into testosterone and androstenedione production. Increased 17 (OH) progesterone or pregnanetriol in the urine is diagnostic of 21 hydroxylase deficiency.

Virilisation may result in clitoromegaly and labial fusion in the female at birth.