Hyponatremia is a condition characterized by low sodium concentration in blood serum. This may be caused by overactivity of anti-diuretic hormone, which results in increased reabsorption of water from the collecting duct. Tolvaptan is an FDA-approved antagonist of the vasopressin V2-receptor. Inhibition of this receptor prevents the expression of aquaporin channels along the collecting duct, promoting diuresis and correcting sodium levels in serum.