Samsca Post Marketing Surveillance Study

Inclusion Criteria: * Subjects must meet all of the following inclusion criteria to be eligible for enrollment into the study : 1. Patients who have hyponatremia in euvolemic or hypervolemic states, defined as serum sodium level \< 125 mEq/L or hyponatremia that is symptomatic and has resisted correction with fluid restriction 2. Patients who are prescribed Samsca® treatment as per investigator's medical judgment 3. Patients who gave written authorization to use their personal and health data 4. Patients starting Samsca® treatment after agreement is in place Investigators will refer to the product market authorization (label) for inclusion criteria. Exclusion Criteria: * Subjects presenting with any of the following will not be included in the study: 1. Patients who have been treated with Samsca® 2. Patients with known or suspected hypersensitivity to tolvaptan or to any ingredient of the drug 3. Patients requiring urgent intervention to raise serum sodium acutely. 4. Inability of the patient to sense or appropriately respond to thirst. 5. Hypovolemic hyponatremia 6. Concomitant use of strong CYP3A inhibitors 7. Anuric patients 8. Volume depletion patients 9. Hypernatremia patients 10. Women who are pregnant or possibly pregnant and lactation 11. Patients with rare hereditary problems of galactose intolerance, the Lapp lactase deficiency or glucose-galactose malabsorption Investigators will refer to the product market authorization (label) for exclusion criteria.