SAT-003 - When Hypophysitis Is Not the Answer: Isolated Pituitary Stalk Langerhans Cell Histiocytosis in an Adult

Langerhans cell histiocytosis (LCH) is an uncommon idiopathic disorder, defined by the clonal expansion of Langerhans-type histiocytes with multisystem involvement. Fewer than 30% of cases occur in adults, with an estimated incidence of 1–2 cases per million per year. Isolated involvement of the hypothalamic–pituitary region is exceptionally rare. We report the case of a 32-year-old woman with a progressive pituitary stalk lesion initially presenting as arginine vasopressin deficiency and evolving over more than three years to a definitive diagnosis of LCH. In May 2022, the patient presented with polyuria and polydipsia and was diagnosed with arginine vasopressin deficiency, requiring desmopressin therapy. Initial pituitary MRI demonstrated mild, diffuse stalk thickening without nodularity, preserved signal intensity, and a normal adenohypophysis, suggesting an inflammatory etiology. Baseline pituitary function was preserved, and extensive systemic, autoimmune, granulomatous, and neoplastic evaluations were negative. Investigations excluded sarcoidosis, IgG4-related disease, and Langerhans cell histiocytosis; autoimmune and tumor markers (including ANA, IgG4, AFP, β-hCG, and ACE) were negative. Chest and abdominal CT scan was unremarkable.
Over subsequent months, the patient developed oligomenorrhea accompanied by hyperprolactinemia. Cabergoline normalized prolactin levels; however, amenorrhea persisted. Gonadotropins were inappropriately normal in the setting of low estradiol, consistent with hypogonadotropic hypogonadism, while thyroid and adrenal axes remained intact. Repeated MRI showed slight increase in pituitary stalk lesion. 18F-FDG PET/CT showed persistent focal hypermetabolism confined to the pituitary stalk, without systemic disease. Given the clinical picture and imaging findings, autoimmune lymphocytic hypophysitis was initially favored, and a trial of glucocorticoid therapy was instituted. Serial MRIs revealed progressive enlargement of the pituitary stalk with homogeneous contrast enhancement. Cerebrospinal fluid studies, tumor markers (AFP, β-hCG), and repeated systemic workups remained negative.
Despite the gradual development of additional pituitary deficits (secondary hypothyroidism), the lesion remained anatomically confined to the stalk on serial imaging.
Following multidisciplinary discussion, a transsphenoidal pituitary stalk biopsy was performed in September 2025. Histopathology showed CD1a- and S100-positive histiocytic infiltrates, and molecular analysis identified a BRAF V600E mutation, confirming LCH. The patient was treated with stereotactic radiotherapy (20 Gy in 10 fractions) without complications.
This case underscores the diagnostic challenges of isolated pituitary stalk lesions and highlights the importance of long-term surveillance and tissue diagnosis in progressive cases.

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