Normal sodium levels in the extracellular fluid are in the range of 135–145 mmol/L. Low serum sodium (hyponatraemia) is a common water and electrolyte balance disorder. Dehydration and overhydration are the most common causes of hyponatraemia in children. This paper describes a case of a 2.5-year-old boy admitted to the paediatric ward due to pneumonia. The child showed reluctance to fluid intake, hypoosmolar hyponatraemia with hypouricaemia, and increased urinary sodium excretion in the absence of abnormalities in hydration status. He had a family history of persistent hyponatraemia and hypertension in his grandfather. After excluding pituitary, thyroid, adrenal cortex pathologies, kidney disfunction, and proliferative processes in the differential diagnosis, cerebral salt wasting syndrome, the syndrome of inappropriate secretion of antidiuretic hormone and the renal-related syndrome of inappropriate antidiuresis were taken into account. The aim of the study was to discuss the differential diagnosis of chronic euvolemic hyponatraemia and to draw attention to the need for genetic testing for the nephrogenic syndrome of inappropriate antidiuretic hormone secretion caused by activating point mutations of the vasopressin 2 receptor gene (V2R).

hyponatraemia, nephrogenic syndrome of inappropriate antidiuresis (NSIAD), syndrome of inappropriate antidiuretic hormone secretion (SIADH), cerebral salt wasting syndrome (CSWS), children