Chronic hyponatraemia with a reset osmostat: when abnormal is normal

Hyponatraemia is one of the most common electrolyte abnormalities encountered in clinical practice, seen in up to 30% of hospitalised children. A key part of the evaluation of hyponatraemia concerns the assessment of volume status. Whereas hypovolaemia indicates a deficit in salt, euvolaemic hyponatraemia is most commonly associated with excess water. An important yet often forgotten differential diagnosis in chronic euvolaemic hyponatraemia is a reset osmostat, where patients regulate their plasma osmolality around a set point below the normal range. Here we present a case of an adolescent with chronic hyponatraemia and how we arrived at a diagnosis of a reset osmostat through simple diagnostic tests.