Early-Onset Hyponatremia Presenting as Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) Post-transsphenoidal Pituitary Resection.

Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a disorder in which an excessive amount of antidiuretic hormone (ADH) is released, leading to water retention and subsequent hyponatremia. Delayed hyponatremia, typically beginning on postoperative day (POD) 4, is a well-recognized complication following transsphenoidal resection of pituitary tumors and is thought to result from excess release of stored ADH due to surgical trauma. In this rare case of early symptomatic hyponatremia secondary to SIADH, a 61-year-old postmenopausal woman with no significant past medical history developed symptomatic hyponatremia on POD 1 after elective transsphenoidal resection of a nonfunctioning pituitary neuroendocrine tumor. Her serum sodium reached a nadir of 126 mmol/L on POD 2. Urine osmolality was 961 mOsm/kg on POD 1, consistent with SIADH. She was treated with fluid restriction, intravenous 3% hypertonic saline, and salt tablets, with improvement in serum sodium prior to discharge on POD 4 on fluid restriction and salt tablets. This case highlights the importance of early clinical vigilance for hyponatremia following pituitary resection, as symptomatic hyponatremia can rarely occur in the early postoperative period.