Urea for Treatment of Hyponatremia in Patients With SIAD Unresponsive to Fluid Restriction

Inclusion Criteria: * Confirmed diagnosis of SIAD established prior to study inclusion, defined as: * Plasma sodium concentration \<135 mmol/L * Plasma osmolality \<300 mOsm/kg * Urine osmolality \>100 mOsm/kg * Urine sodium concentration \>30 mmol/L * Intact adrenal and thyroidal function and no use of diuretics in the last 4 weeks * Clinical euvolemia, defined as an absence of signs of hypovolemia (orthostasis, tachycardia, decreased skin turgor, dry mucous membranes) or hypervolemia (edema, ascites) Fulfillment of all diagnostic criteria at baseline (day 0) was not mandatory, as the diagnosis of SIAD had already been established prior to study inclusion based on overall clinical assessment. \- Written informed consent obtained Exclusion Criteria: * Chronic hypotonic hyponatremia secondary to another etiology * Presence of moderate to severe symptoms attributable to plasma hypotonicity * Severe hypotonic hyponatremia (serum sodium \<120 mmol/L) * Another ongoing drug treatment for hyponatremia (including vaptans and salt tablets) * Severe hepatic insufficiency * eGFR \<45 mL/min/1.73 m² * Pregnancy or breastfeeding * Known allergy or intolerance to urea * Patient refusal or inability to provide written informed consent