Diagnosing reset osmostat is a diagnosis of exclusion. Individuals must be euvolemic, and a thorough exclusion of other causes of euvolemic hyponatremia (e.g., hypothyroidism, cortisol deficiency, medications, etc) must take place. A key feature of reset osmostat is that individuals should be able to concentrate and dilute the urine appropriately. Thus, a water challenge should result in a dilute urine (e.g., less than 100 mOsm/kg) and a water deprivation test should result in a concentrated urine. Sometimes, a patient given a diagnosis of SIADH will be proven to be reset osmostat when it becomes apparent that fluid restriction does not successfully raise the serum sodium level.
Reset osmostat classically occurs in neurologic conditions such as epilepsy and paraplegia, in addition to pregnancy, malignancy, and malnutrition. It has also been observed in healthy individuals, such as this 60 year-old man with a chronic sodium level between 125-130 mmol/L; the authors suggest that a 1951 grenade explosion the patient experienced may have caused the osmostat to reset!