Beyhan GÜVERCİN, Berk Buğra ZAMAN, Melike CAN, Şükrü ULUSOY
Medical Science and Discovery - 2026;13(3):45-50
Objective: Reset osmostat (RO) is an under-recognized disorder of osmoregulation characterized by a downward resetting of the plasma osmolality threshold for arginine vasopressin release. Affected individuals develop chronic, typically mild and stable hypotonic hyponatremia while preserving the ability to dilute urine and excrete a water load. Because RO is frequently misclassified as the syndrome of inappropriate antidiuresis (SIAD), patients may undergo unnecessary investigations and receive potentially harmful therapies aimed at normalizing serum sodium. To summarize the physiology of the osmostat, identify clinical contexts associated with RO -including congenital midline brain malformations -and highlight practical diagnostic features that distinguish RO from SIAD and other causes of euvolemic hyponatremia. Material and Methods: This narrative review is based on targeted literature searches of PubMed and manual review of reference lists, with emphasis on osmoregulatory physiology, diagnostic strategies, and reported associations with congenital midline brain abnormalities. An illustrative case of chronic asymptomatic hyponatremia is also presented. Conclusions: RO represents a regulated but abnormally set osmoregulatory state in which vasopressin secretion occurs at a lower plasma osmolality threshold. Key diagnostic features include chronic, stable hyponatremia, preserved urinary dilution, normal fractional excretion of urate, and limited response to SIAD-directed therapies. RO has been reported in association with central nervous system disorders, particularly congenital midline malformations such as agenesis of the corpus callosum. Recognition of RO is essential to avoid misdiagnosis and inappropriate treatment. Management is typically conservative, with emphasis on avoiding unnecessary correction of serum sodium and preventing overly rapid correction when superimposed acute processes occur.
