The registry is a multi-center, prospective, observational study designed to collect data in patients with euvolemic and hypervolemic hyponatremia in the hospital setting in the USA and hyponatremia secondary to syndrome of inappropriate antidiuretic hormone secretion (SIADH) in Europe; patients with hypovolemic hyponatremia will be excluded from the registry. The registry is observational in design, and the registry will collect data on patients with a serum sodium ≤130 mmol/L. Data from the patients' chart will be collected throughout the duration of the hospital, from admission to discharge, and the data will be entered into the case report form (CRF) or into an electronic data capture (EDC) system. These patients will be entered into the registry over the duration of 18 months at community, tertiary, and academic medical centers from diverse multiple regions of the USA and Europe.

A patient's treatment will be determined by their caregiver and not by this protocol. Thus, the registry will provide a unique opportunity to evaluate how the patients are managed under real-world management conditions.

Inclusion Criteria:

• Adults ≥18 years old who are hospitalized

• Euvolemic or hypervolemic hyponatremia with serum sodium ≤130mmol/L

• For euvolemic hyponatremia:

  • Euvolemia is defined as: absence of clinical and historical evidence of extracellular fluid volume depletion or sequestration; and absence of edema and ascites; or
  • Physician diagnosis of SIADH

• For hypervolemic hyponatremia (applies to USA sites only):

  • Hypervolemia is defined as: excess extracellular fluid volume manifesting as dependent edema or ascites

  • Patients may have one or more of the following underlying co-morbid conditions:

    • Congestive heart failure
    • Cirrhosis and/or liver failure
    • Nephrotic syndrome

Exclusion Criteria:

• Patients with hypovolemic hyponatremia
• Use of investigational drug, biologic, or device during the study

The prospective registry study population will consist of up to 1500 patients at approximately 150 sites from the USA who are hospitalized and identified as having hypervolemic or euvolemic hyponatremia with serum sodium ≤130 mmol/L. Also up to 1005 patients at approximately 67 sites from Europe (United Kingdom, Germany, France, Italy, Spain, Denmark, and Sweden)who are identified as having hyponatremia ≤130mmol/L secondary to SIADH. A similar distribution of patients with a serum sodium <125 mmol/L and patients with a serum sodium between 125-130 mmol/L will be collected.