SUN-002 Lymphocytic Infundibulo-neurohypophysitis With Unusual Course Of Desmopressin-resistant Diabetes Insipidus

Abstract Disclosure: A. Garcia-Bautista: None. J. Panneflek: None. J. Garza: None. M. Twahirwa: None. M. Castillo: None. Introduction: Lymphocytic hypophysitis is a rare condition affecting the pituitary gland, causing hypopituitarism due to inflammation or a sellar mass lesion. It accounts for about two-thirds of primary hypophysitis cases and is more common in females, with a female-to-male ratio of 2-4:1. Patients often have a history of autoimmune diseases. Treatment involves observation for mild to moderate cases, while moderate to severe cases are typically treated with glucocorticoids. Case report A 43-year-old female with rheumatoid arthritis, diabetes mellitus presented with one month of worsening polyuria and polydipsia. She reported urinating >10 times daily and waking 4 times nightly. Associated symptoms included dry mouth, weakness, fatigue, and headaches which affected her workout routine. Nausea, tremors, and poor oral intake led to an emergency room visit. She denied head trauma or surgery. Medications included metformin, semaglutide, and adalimumab (last dose six months ago). She denied use of lithium, supplements, tobacco, alcohol, or drugs. Initial labs: sodium 150 mEq/L, serum osmolality 309 mOs/kg, urine osmolality 119 mOs/kg, specific gravity 1.003, ACTH 15 pg/mL, cortisol (4 PM) 4.4 µg/dL, prolactin 35.8 ng/mL, TSH 0.79 µIU/mL. Brain MRI showed an abnormal pituitary signal with loss of the normal T1 bright spot, hyperintense T2/FLAIR signal extending into the infundibulum, and post-contrast enhancement. The infundibulum measured 0.3 cm thick, and the gland measured 1 × 1 cm, without macroadenoma or optic chiasm compression. Gutenberg score: -5. Treatment began with oral and IV DDAVP, which was ineffective. Prednisone was subsequently initiated at 40 mg daily along with intranasal DDAVP (0.01%), resulting in resolution of polyuria within a day. After 10 days, prednisone was increased to 50 mg daily, leading to improved symptoms and stabilization of serum sodium at 134 mEq/L. Discussion: This case presents an atypical manifestation of lymphocytic infundibulo-neurohypophysitis, characterized by desmopressin-resistant central diabetes insipidus, in a patient with associated autoimmune comorbidities. Despite an initial lack of response to oral and intravenous DDAVP, the patient's symptoms demonstrated significant improvement following glucocorticoid therapy and intranasal DDAVP administration. This observation underscores the critical importance of early identification and tailored immunosuppressive treatment in managing these atypical cases. Presentation: Sunday, July 13, 2025