Arginine vasopressin deficiency revealing a disseminated human chorionic gonadotropin-secreting germinoma.

Arginine vasopressin deficiency (AVP-D) is an uncommon disorder caused by deficient vasopressin production due to hypothalamic or pituitary gland dysfunction resulting in hypotonic polyuria, whose etiology oftentimes remains elusive. We present the case of a 20-year-old male patient evaluated for polyuria and polydipsia. Laboratory evaluation demonstrated hypernatremia, elevated serum osmolality, and inappropriately dilute urine with symptomatic improvement following initiation of desmopressin, consistent with AVP-D. Initial pituitary magnetic resonance imaging (MRI) showed a small enhancing lesion with pituitary stalk thickening (PST), raising concern for inflammatory or infiltrative etiologies. Extensive autoimmune and pituitary infiltrative disease work-up was unrevealing, and serial imaging remained inconclusive. Given diagnostic uncertainty, a transsphenoidal pituitary biopsy was performed, which proved consistent with a germinoma with evidence of human chorionic gonadotropin secretion. Subsequent staging studies, including spinal MRI and cerebrospinal fluid analysis, raised concern for possible disseminated disease. The patient was treated with craniospinal irradiation and continues close multidisciplinary follow-up for hypopituitarism and AVP-D. This case highlights the need for continued surveillance in patients presenting with AVP-D and nonspecific imaging findings, as early identification of etiology is critical for timely treatment and optimal outcomes.