Limitations of 17-hydroxyprogesterone in investigating neonatal hyponatraemia

Congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency is a possible cause of hyponatraemia in the neonate. Elevated 17-hydroxyprogesterone (17-OHP) is considered diagnostic of the condition, although there have been reports of anomalous high concentrations, up to 110 nmol/L, in premature, sick infants subsequently shown to have normal adrenocortical function. We describe a case of a 6-week-old girl with a chest infection and hyponatraemia whose plasma 17-OHP concentration was 300 nmol/L, well within the range associated with 21-hydroxylase deficiency. However, there was no genital ambiguity and plasma cortisol was also significantly elevated, raising the possibility of generalized adrenal hyperstimulation rather than CAH. The patient was treated with antibiotic and saline infusions but no steroids. CAH was subsequently excluded by normal 17-OHP and cortisol responses to Synacthen stimulation. In sickness, an increased plasma 17-OHP concentration may not be synonymous with 21-hydroxylase deficiency, even when grossly raised. Simultaneous measurement of plasma cortisol could aid interpretation and avoid potential misdiagnosis, especially in male infants.