Central Diabetes Insipidus Unmasked: A Rare Presentation of Lymphocytic Infundibuloneurohypophysitis

Lymphocytic Infundibulo-Neurohypophysitis (LINH) is a rare autoimmune inflammatory disorder selectively involving the posterior pituitary and stalk, commonly presenting as Central Diabetes Insipidus (CDI). Due to its non-specific clinical and radiologic features, it remains underdiagnosed and is frequently misclassified as idiopathic CDI. We report a 21-year-old female who presented with polyuria (8-10 L/day) and polydipsia (10-12 L/day) over one month, associated with weight gain and headache. Laboratory evaluation revealed normal glycaemia, serum sodium of 139 mEq/L, urine specific gravity of 1.005, and serum osmolality of 287 mOsm/kg. Water deprivation testing demonstrated a rise in serum sodium to 147 mEq/L, serum osmolality to 305 mOsm/kg, and 3 kg weight loss with inadequate urinary concentration, confirming CDI. MRI pituitary showed posterior pituitary T1/T2 hypointense lesion measuring 7.3×4.3×7.0 mm with stalk thickening (5 mm) and absent posterior pituitary bright spot. Differential diagnoses including tuberculosis, IgG4-related hypophysitis, lymphoma, and infiltrative disorders were considered. Biopsy and anti-rabphilin-3A antibody testing were deferred due to financial constraints. A diagnosis of CDI secondary to LINH was made based on clinicoradiologic correlation. The patient was treated with desmopressin and oral glucocorticoids with marked symptomatic and radiologic improvement on follow-up. This case highlights the role of non-invasive diagnostic strategies and conservative management in LINH in resource-limited settings.