Tolvaptan is a vasopressin receptor antagonist. Vasopressin (a.k.a. ADH or antidiuretic hormone) helps to regulate water retention by absorbing water in the collecting ducts of the nephron. Blocking this receptor will allow water to be excreted more readily. Many heart failure patients present with some degree of hyponatremia from water retention. Tolvaptan has been shown in more than one clinical trial to raise sodium levels. However, mortality and rehospitalization was not improved, and thus the role for this therapy is not well defined.