Reset Osmostat: A Challenging Case of Hyponatremia

Hyponatremia is the most common electrolyte abnormality seen in hospitalised patients with up to 15–20% of patients having a sodium level of less than 135 mmol/L (Reddy and Mooradian, 2009). Cases of hyponatremia were first described in the 1950s (George et al., 1955). As the differential diagnosis for hyponatremia is broad, a systematic and logical approach is needed to identify the cause. We describe a case of a 30-year-old gentleman who was found to have chronic hyponatremia. After a thorough workup, he was diagnosed to have reset osmostat. Reset osmostat is an uncommon and under recognised cause of hyponatremia which does not require any treatment. This diagnosis needs to be considered when the hyponatremia workup suggests SIADH, but the hyponatremia is not amenable to fluid restriction, salt or urea supplementation, and diuretic treatment.