The “new normal” osmotic threshold: Osmostat reset

Hyponatremia is the most common electrolyte disorder in hospitalized patients. The syndrome of inappropriate antidiuresis (SIAD) is one of the leading causes of hyponatremia. Although not widely known, SIAD has a vast spectrum of etiologies and differential diagnoses and has been classically divided into four types (A, B, C, D). Frequently, when we use the term SIAD in clinical practice, it refers to subtype A, the so-called classic SIAD. The purpose of reporting this case is to make the clinicians aware of a specific subtype of SIAD, type C, an underdiagnosed entity called osmostat reset (OR). Due to similarities, OR often ends up being misinterpreted as classic SIAD. However, the differentiation between these two entities is crucial due to treatment implications. This manuscript highlights the use of an algorithm, based on the fraction of uric acid excretion, as an approach to the differential diagnosis of hyponatremia.