Nephrogenic syndrome of inappropriate antidiuresis (NSIAD) is an X-linked disease caused by activating mutations in the arginine vasopressin (AVP) receptor-2 (AVPR2) gene. Affected patients excrete concentrated urine despite very low levels of AVP, and consequently develop euvolemic hyponatremia. Due to its low frequency, patients may be misdiagnosed and treated incorrectly. We report two related male infants with NSIAD that was initially confused with hyporeninemic hypoaldosteronism (HH). First, a 2-month-old male presented with hyponatremia, low plasma osmolality, relatively high urine osmolality, and low plasma renin-aldosterone levels. These clinical and laboratory findings were compatible with syndrome of inappropriate antidiuretic hormone secretion without apparent cause. Consequently,fludrocortisone was initiated with a presumptive diagnosis of HH While correction of hyponatremia, fludrocortisone treatment led to hypertension and discontinued in a short time. The second patient at age of 1 year was admitted with a history of oligohydramnios, four times hospitalizations due to hyponatremia since birth, and a diagnosis of epilepsy. Similarly, the second infant had clinical and
laboratory findings compatible with syndrome of inappropriate antidiuretic hormone secretion with no apparent cause. Fluid restriction normalized his serum sodium despite plasma AVP level was undetectable. In both infants, AVPR2 gene analysis revealed a known mutation (c.409C>T; p.R137C) and confirmed the diagnosis of NSIAD. In conclusion, NSIAD should be considered in all patients with unexplained euvolemic hyponatremia despite high urine osmolality. In case of unawareness from NSIAD, plasma renin-aldosterone profile can be confused with HH, especially in the infants.