Protein Supplementation in Hyponatremia Due to the Syndrome of Inappropriate Antidiuresis

Inclusion Criteria: * Adult patients with confirmed SIAD during the hospital stay or at screening, defined by: * Plasma sodium concentration \<135 mmol/L * Plasma osmolality \<300 mOsm/kg * Urine osmolality \>100 mOsm/kg * Urine sodium concentration \>30 mmol/L * Clinical euvolemia (no signs of hypovolemia or hypervolemia) Exclusion Criteria: * Lactose intolerance, milk protein allergy, soy allergy, nuts allergy, or hypersensitivity to components of the protein supplement. * Inborn metabolic disorders affecting carbohydrate, lipid, or protein metabolism. * Severe symptomatic hyponatremia requiring 3% NaCl or intensive care. * New (within the last five days) treatment with SGLT2 inhibitors, vaptans or oral urea * Uncontrolled severe hypothyroidism (untreated) * Uncontrolled adrenal insufficiency (morning cortisol \<150nmol/l) * eGFR \<45 mL/min/1.73 m² (KDIGO G3b-5) or end-stage renal disease (dialysis) * Severe hepatic impairment or advanced symptomatic liver disease defined as past or current hepatic encephalopathy, liver cirrhosis Child C, or decompensated (bleeding, jaundice, hepatorenal syndrome) * Pregnancy, breastfeeding, or plans to become pregnant during the study. * End-of-life care * Lack of capacity or other reasons preventing from giving informed consent or following study procedures (e.g., due to language problems, psychological disorders, dementia, etc.) * Treatment with thiazid diuretic (must be stopped at least 48 hours before inclusion) Post-randomization Exclusion Criteria: * Type 1 diabetes or uncontrolled type 2 diabetes (HbA1c \>8.0%)