• Jingyan Li, Xinxin Zhang, Qing He, Wenli Feng, Li Ding, Zhuoqun Wang, Haonan Yu, Qiusong Chen, Ning Lu, Dongbo Xu, and Jingqiu Cui.
• Department of Endocrinology and Metabolism, Tianjin Medical University General Hospital, Tianjin, China.
• Medicine (Baltimore). 2021 Oct 8; 100 (40): e27453e27453.

RationaleFunctional pancreatic neuroendocrine tumors (pNETs) rarely produce vasopressin. Here, we reported a case of pNET producing vasopressin in a 78-year-old man with hyponatremia.Patient ConcernsThe patient presented with anorexia approximately 4 years ago, and the laboratory test results indicated hyponatremia. He was hospitalized 3 times subsequently due to anorexia in the past 4 years, during which laboratory tests consistently indicated severe hyponatremia.DiagnosisUpon admission, his serum osmolarity, urine osmolarity, urine sodium level, and 24-hour urine sodium level was 277 mOsm/kg H2O, 465 mOsm/kg H2O, 82.5 mmol/L, and 140.25 mmol, respectively. Gallium-68-labeled tetraazacyclododecanetetraacetic acid-Dphel-Tyr3-octreotate positron emission tomography-computed tomography showed a high uptake lesion measuring approximately 1 cm in diameter in the pancreatic body, and the possibility of pNET was considered. Besides, laboratory tests showed that adrenocorticotropic hormone, follicle-stimulating hormone, and luteinizing hormone released by the pituitary was insufficient in the case of low levels of cortisol, estradiol, progesterone, and testosterone. Thus, the diagnosis of the syndrome of inappropriate antidiuresis (SIAD) was considered along with hypopituitarism.InterventionsThe patient underwent surgery, and pNET was confirmed by pathology examination. The immunohistochemical study showed that the tumor cells were positive for somatostatin receptors 2 and vasopressin.OutcomesIn the last follow-up 17 months after surgery, the patient was in good condition, taking methylprednisolone 4 mg every other day, and had been free of anorexia or hyponatremia episodes.LessonsThis case illustrated the potential ectopic production of vasopressin resulting in SIAD in pNETs, highlighting the adoption of gallium-68-labeled tetraazacyclododecanetetraacetic acid-Dphel-Tyr3-octreotate positron emission tomography-computed tomography and vasopressin immunohistochemical staining in the evaluation of the etiology of SIAD.Copyright © 2021 the Author(s). Published by Wolters Kluwer Health, Inc.

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