AVP and Hyponatremia: Hyponatremia in the early postoperative period [before postoperative day (POD) 3] is mostly attributed to improper fluid management (5), while delayed postoperative hyponatremia (DPH) usually results from the syndrome of inappropriate secretion of antidiuretic hormone (SIADH) or hypocortisolemia, rarely from cerebral salt-wasting syndrome (6, 7).