Not Noncompliant, Just Reset: A Case of Reset Osmostat

Introduction: Reset osmostat (RO) is a lesser-known cause of chronic hyponatremia in which hypothalamic osmoreceptors regulate vasopressin (ADH) release at a lower serum sodium(Na) set point. RO mimics the syndrome of inappropriate antidiuresis (SIADH) but does not typically lead to severe hyponatremia and often requires no treatment. It is frequently overlooked, especially in patients labeled as non-compliant with fluid restriction. We describe a patient ultimately diagnosed with RO after initial misattribution of her hyponatremia to non-compliance. Case Description: A 60-year-old woman with alcohol use, and hypothyroidism presented after a fall and was found to have a right shoulder fracture. Admission labs showed acute kidney injury (Creatinine 1.4 mg/dL, baseline 0.5 mg/dL) and serum sodium of 118 mmol/L. Urine Na was 19 mmol/L and urine osmolality (Uosm) was 286 mosm/kg, suggesting volume depletion. She received IV fluids, with resolution of the AKI and partial correction of sodium to 122–126 mmol/L. Despite ongoing fluid restriction and increased solute intake, sodium remained persistently low. Intermittent low Uosm prompted concern for RO. Due to suspicion of poor adherence, the patient was questioned repeatedly, though she firmly denied excessive water intake. A water loading test was then performed. After administration of 10 mL/kg of IV D5W induced appropriate urinary dilution, a diagnosis of RO was made. The figure below highlights the changes with water loading. Her sodium level then stabilized between 125–130 mmol/L without further intervention. Discussion: RO is a diagnosis of exclusion and should be considered in patients with persistent hyponatremia resembling SIADH, particularly when standard interventions fail. In RO, ADH secretion remains osmoregulated but at a lower sodium threshold. Recognition of RO can prevent unnecessary treatment, reduce hospital stay, and avoid inappropriate labeling of patients as non-compliant. Water loading remains the gold standard for diagnosis but must be done cautiously to avoid worsening hyponatremia.