Introduction Nephrogenic diabetes insipidus is a rare X linked syndrome, secondary to mutations in the gene encoding the vasopressin V2 receptor, located at position Xq28. In infants, clinical presentation is with signs of dehydration (fever, constipation, vomiting, failure to thrive and developmental delay).

Aim To present a rare cause for failure to thrive in infancy, which undiagnosed can lead to poor outcome.

Material and method We report a 4 month old patient, male admitted in our hospital for growth impairment, vomiting and fever. Results: Family history (uncle with poliuro-polidipsic syndrome, whose brother died at the age of two with severe hypernatremic dehydration), polyuria 7.8 ml/kg/h, lab work-up (plasma hyperosmolarity, hypernatremic hyperchloremic alkalosis, extremely low urine density (1002), normal ADH levels) were suggestive for nephrogenic diabetes insipidus. Treatment with hydrochlorothiazide was initiated (2 mg/kg/body), with reduction in urine output (3.5 ml/kg/h), but severe consecutive alkalosis and hypokalemia. An aldosterone antagonist (spironolactone) was added, with good response.

Discussion The diagnosis is usually delayed until hypernatremic dehydration occurs. Treatment is symptomatic, difficult in infants. It aims to avoid episodes of dehydration and obtain a normal growth curve.

Conclusions This highlights the importance of early diagnosis of diabetes insipidus in infants with persistent hypernatremia.