Langerhans cell histiocytosis (LCH) may present as unifocal disease of the suprasellar region, with symptoms and signs of hypopituitarism, arginine vasopressin deficiency (AVP-D) and weight gain. Transcranial biopsy is necessary, to define diagnosis and guide treatment decisions, but is associated with significant morbidity. We describe a patient with Hashimoto thyroiditis and a single hypothalamic mass in whom LCH diagnosis was done through thyroid fine-needle aspiration cytology (FNAC) performed despite nonspecific findings in thyroid imaging, on the basis of a slightly elevated [18F]-fluorodeoxyglucose avidity on positron emission tomography/-computed tomography (FDG-PET/-CT), and volume increase during follow-up.