Hyponatremia is a common complication following endoscopic endonasal resection (EER) of pituitary adenomas. We report a single center, multi-surgeon study detailing baseline clinical data, outcomes, and factors associated with postoperative hyponatremia. A retrospective cohort study of patients undergoing EER for pituitary adenoma at Tufts Medical Center was conducted. Most procedures were performed by the senior author (CBH). Cases were included if at least one postoperative sodium value was available and pathology confirmed pituitary adenoma. Hyponatremia was considered postoperative sodium <135 mEq/L. 272 patients underwent 310 EER procedures that met study criteria. Mean age was 53.3 years and mean tumor size was 18.8 mm. Postoperative hyponatremia occurred in 12.6% of cases, with 3.6% developing hyponatremia prior to discharge. Lower pre-operative sodium was associated with increased risk of developing any postoperative hyponatremia. Older age, prolactinoma pathology, and SSRI use were associated with moderate-severe hyponatremia (≤129 mEq/L), while lower preoperative sodium was associated with mild hyponatremia (130-134 mEq/L). Hyponatremia-related readmissions within 30 days occurred in 3.9% of patients. African-American race and postoperative hyponatremia were both associated with an increased risk of 30-day readmission. Mean nadir sodium for hyponatremic patients was 129.9 mEq/L. Growth hormone secreting pathology was associated with lower postoperative nadir sodium, while higher preoperative sodium was associated with higher postoperative nadir sodium. Hyponatremia is a common postoperative complication of EER for pituitary lesions that can cause significant morbidity, increase readmissions, and lead to increased healthcare costs.