Congenital hyperinsulinism characterized by over-secretion of insulin is the most common cause of recurrent hypoglycemia in the neonatal period and can cause irreversible brain damage. In this case hyperinsulinemia was diagnosed by inappropriately elevated insulin levels during hypoglycemia, with negative ketone bodies in the urine and increased glucose levels to glucagon. Hyperammonemia persisted with sodium benzoate therapy and was not affected by protein restriction. Serum and urine amino acids and organic acid levels in urine were not consistent with urea cycle defects or with other hyperammonemia syndromes. A410T mutation was detected in the child and she was diagnosed as hyperinsulinemia hyperammonemia syndrome.

Keywords: hiperinsülinemi, hiperamonemi, hyperinsulinemia, hyperammonemia