Case Report: Central diabetes insipidus 15 years after a clinical diagnosis of granulomatosis with polyangiitis

Background Pituitary involvement occurs in approximately 1% of patients with granulomatosis with polyangiitis (GPA), and arginine vasopressin deficiency (AVP-D; central diabetes insipidus) is a frequent endocrine manifestation. AVP-D may be masked by other causes of polyuria. Case presentation A 35-year-old woman with a 15-year history of a clinical diagnosis of GPA receiving immunosuppression was admitted with 2 months of anorexia. Appetite loss preceded a recent zoledronic acid infusion, after which further worsening was reported. Hypernatremia (peak: 157.1 mmol/L) was ascribed to poor intake, and polyuria was confounded by diabetes mellitus and suspected urinary infection. The admission urine specific gravity of 1.008 lay within the reference range yet was inappropriate for the concurrent hypernatremia; after rehydration, glycemic control, and antibiotics, hypotonic polyuria persisted. Plasma and urine osmolality were 295 and 70 mOsm/kg before desmopressin, and urine output fell markedly after oral desmopressin, supporting AVP-D. During a monitored oral desmopressin trial, 4-h urine output fell by 77.1% relative to the mean for the same interval on the preceding two observation days. Low free thyroxine with nonelevated TSH suggested possible central hypothyroidism. ANCA tests were negative in 2023 and 2026; results from the initial diagnosis were unavailable. Unenhanced sellar MRI showed a bulky pituitary containing a 7.6 mm × 6.1 mm lesion. Multidisciplinary review favored GPA-related posterior pituitary enlargement. The cause remained uncertain without histological confirmation or contrast-enhanced follow-up MRI, and histiocytic disorders could not be excluded. At telephone follow-up, the patient reported symptom control with desmopressin 0.1 mg twice daily. Conclusion AVP-D can be masked by competing explanations for hypernatremia and polyuria. A specific gravity within the reference range does not exclude AVP-D when hypernatremia coexists. Attribution of the sellar lesion requires further investigation.