Study of the Safety and Effectiveness of SAMSCA® (Tolvaptan) in Children and Adolescents With Euvolemic or Hypervolemic Hyponatremia

Inclusion: * Male and female participants ≥4 weeks (or ≥44 weeks adjusted gestational age) to \<18 years old * Participants hospitalized with euvolemic or hypervolemic hyponatremia resistant to initial standard background therapy * Persistent euvolemic or hypervolemic hyponatremia defined as being documented as \<130 milliequivalent (mEq)/L and present for at least 48 hours, evidenced by at least 2 serum sodium assessments (12 hours apart) * Ability to maintain adequate fluid intake (orally or intravenously) * Ability to take oral medications * Ability to comply with all requirements of the trial * Completion of the trial-specific informed consent/assent as age appropriate * Ability to commit to remain fully abstinent or practice double-barrier birth control as required by the trial Exclusion: * Evidence of hypovolemia or intravascular volume depletion * Serum sodium \<120 mEq/L * Use of potent cytochrome P450 3A4 (CYP3A4) inhibitors in participants \<12 kilogram (kg) or moderate CYP3A4 inhibitors in participants \<6 kg * Lacks free access to water (inability to respond to thirst) or without intensive care unit level fluid monitoring and management * History or current diagnosis of nephrotic syndrome * Transient hyponatremia likely to resolve * Hyperkalemia * Estimated glomerular filtration rate \<30 milliliters/minute/1.73 meters squared * Acute kidney injury * Severe or acute neurological symptoms requiring other intervention * Prior treatment for hyponatremia with hypertonic saline within 8 hours of qualifying serum sodium assessments; urea, lithium, demeclocycline, conivaptan, or tolvaptan within 4 days of qualifying serum sodium assessments; any other treatments for the purpose of increasing serum sodium concurrent with dosing of trial medication * Anuria or urinary outflow obstruction, unless participant is/can be catheterized * History of hypersensitivity and/or idiosyncratic reaction to benzazepine or benzazepine derivatives * Psychogenic polydipsia * Uncontrolled diabetes mellitus (defined as fasting glucose \>300 milligrams/deciliter) * Screening liver function values \>3 times the upper limit of normal * Participants who have cirrhosis and meet any of the following conditions: a major GI bleed within the past 6 months, evidence of active bleeding, platelet count \<50,000/microliter, or use of concomitant medications known to increase bleeding risk * Hyponatremia due to the result of any medication that can safely be withdrawn or that is most appropriately corrected by alternative therapies * History of drug or medication abuse within 3 months prior to screening or current alcohol abuse * Participants who require suspension formulation and have a Hereditary Fructose Intolerance * Has hyponatremia that is more appropriately corrected by alternative therapies * Is pregnant or currently breastfeeding * Has any medical condition that could interfere with evaluation of trial objectives or participant safety * Has participated in another investigational drug trial in the last 30 days * Weighs \<3 kg * Unable to swallow tablets, if suspension unavailable * Is deemed unsuitable for trial participation in the opinion of the investigator