Subjects will be randomized, with stratification by baseline sodium <130 or ≥ 130 mEq/L[mmol/L] to receive either tolvaptan 15 mg tablet or matching placebo tablet at doses of 15, 30 or 60 mg for 21 days. During this period, fluid restrictions should be loosened or suspended, until the subject's response to therapy can be evaluated, typically over the first few days of therapy. Fluid restriction may be reinstituted at any time in subjects whose sodium fails to improve or worsens with study therapy. A forced-titration up to 60 mg of study drug by day 3 to 7 will be based on the subject's serum sodium Subjects entering the study with a serum sodium concentration less than 130 mEq/L[mmol/L] may be fluid restricted if necessary at the discretion of the Investigator. Subjects should be monitored closely during the first 24 hours of treatment for dosing titration. The total dosing duration will be up to 21 days (plus 3 day treatment window). Subjects will return to the clinic on Day 22 (+3 days) for assessments and will complete a follow-up visit on Day 28 (+2 days).