Reset or inappropriate? Reset osmostat vs SIADH: A Clinical perspective on persistent euvolaemic hyponatraemia

Abstract

Persistent euvolaemic hyponatraemia often presents a diagnostic dilemma, particularly when laboratory findings resemble the syndrome of inappropriate antidiuretic hormone secretion (SIADH). Reset osmostat represents an important alternative diagnosis in which the osmoregulatory system remains intact but the threshold for vasopressin release is shifted to a lower level, resulting in a stable but reduced serum sodium concentration. This presentation provides a practical clinical perspective on distinguishing reset osmostat from SIADH. The discussion focuses on the underlying physiology of vasopressin regulation and free-water excretion, followed by a structured approach to evaluating chronic hypotonic hyponatraemia. Key clinical and biochemical clues, including longitudinal sodium stability, urine osmolality, urine sodium and fractional excretion of urate, are explored. The presentation highlights the contrasting physiological responses to water loading, where preserved water excretion supports reset osmostat, whereas impaired water excretion favours SIADH. Two contrasting clinical scenarios are used to demonstrate how the trajectory of serum sodium and response to conventional treatment can help avoid misclassification. The presentation also addresses the practical implications of diagnosis. While SIADH generally requires identification and treatment of the underlying cause together with appropriate water-balance management, reset osmostat usually does not require attempts to normalise a stable sodium concentration. The potential role and limitations of water-loading testing are discussed, including the need for careful patient selection because of the risk of worsening hyponatraemia. The key message is that chronic euvolaemic hyponatraemia is not synonymous with SIADH. Recognising a reset osmostat requires attention to the clinical trajectory and the physiology of water excretion, allowing clinicians to treat the mechanism rather than simply chase the sodium number. 

[J Assoc Clin Endocrinol Diabetol Bangladesh, 2026;5(Suppl 1): S15]