MON-030 A Case of Idiopathic Arginine Vasopressin Deficiency Diagnosed Following Worsening of Thirst, Polydipsia, and Polyuria During Chemotherapy

Abstract Disclosure: N. Iwata: None. Y. Kato: None. O. Murase: None. T. Tsumura: None. H. Fujisawa: None. Y. Sugimura: None. Case Report Patient: 52-year-old male The patient had experienced frequent urination for several years, with no abnormalities detected in prostate or bladder examinations. In June 2023, he developed narrow stool caliber and hematochezia, leading to a diagnosis of sigmoid colon cancer with multiple liver metastases. Chemotherapy with FOLFOX + Pmab (Fluorouracil, Oxaliplatin, Panitumumab) was initiated in August 2023. From October 2023, he reported dry mouth and increased water intake (8-10 L/day) and urination, prompting referral to our department. Laboratory findings showed serum Na 142 mEq/L, plasma osmolality 284 mOsm/kg H2O, urine osmolality 218 mOsm/kg H2O, and plasma AVP <0.4 pg/mL. Dynamic tests (CRH, TRH, LHRH stimulation tests) and GHRP-2 stimulation test results were normal. However, the hypertonic saline loading test showed low AVP response (0.5 pg/mL → 0.8 pg/mL), leading to the diagnosis of Arginine Vasopressin Deficiency (AVP-D). Anti-rabphilin-3A antibodies were positive. Pituitary MRI showed loss of posterior pituitary bright spot, and a 3 mm cystic lesion considered to be a Rathke’s cleft cyst. Based on these findings, idiopathic AVP-D was diagnosed. Administration of DDAVP 60 μg/day at bedtime resolved nocturnal polyuria, with urine osmolality improving to 525-746 mOsm/kg H2O. Discussion The patient had mild polydipsia and polyuria prior to chemotherapy, but these symptoms worsened after its initiation, leading to the diagnosis of AVP-D. Causes of AVP-D include tumors, inflammation, and familial disorders. In this case, the small Rathke’s cleft cyst observed on MRI was unlikely to cause AVP-D, and no other structural abnormalities were identified, resulting in a diagnosis of idiopathic AVP-D. Autoimmune mechanisms, such as lymphocytic infundibulo-neurohypophysitis, are thought to contribute to idiopathic AVP-D. We have previously reported that over 30% of idiopathic AVP-D cases are positive for anti-rabphilin-3A antibodies. As for the worsening of diabetes insipidus symptoms after chemotherapy, possible explanations include the effects of steroids used in chemotherapy, undetectable micro-metastases in the hypothalamo-hypophyseal region, or newly induced pituitary dysfunction caused by chemotherapy. Presentation: Monday, July 14, 2025