Loss of therapeutic control in congenital adrenal hyperplasia due to interaction between dexamethasone and primidone

A 14-year-old girl with congenital adrenal hyperplasia showed rapid conversion from undertreatment to overtreatment when primidone, used to treat coexistent epilepsy, was withdrawn while on a constant steroid dose. In addition to signs of hypercortisolism and a decrease in random measurements of plasma testosterone and 170H-progesterone concentrations, the deterioration in control was also illustrated by changes in the pattern of blood spot steroid profiles. Adequate control was achieved only after a 3-fold reduction in steroid dose. The decrease in dose requirement was probably the result of a reversal of liver enzyme induction consequent upon the withdrawal of primidone.