Thyroid gland involvement by Langerhans cell histiocytosis is extremely rare. A 35-year-old woman with a history of a suprasellar mass previously diagnosed as a ganglioglioma and complicated by diabetes insipidus, hypogonadotropic hypogonadism, and central hypothyroidism presented with acute onset of neck enlargement. On ultrasound examination, almost the entire thyroid appeared replaced by abnormal lobulated hypoechoic tissue with increased vascularity. Fine needle aspiration (FNA) of the thyroid was performed and revealed singly scattered and loosely cohesive large cells with abundant cytoplasm, including some with irregular nuclear contours and nuclear grooves. No thyroid follicular cells were noted. Based on the cytomorphologic findings and ancillary studies (immunohistochemistry and flow cytometry analysis) a cytological diagnosis of "positive for neoplastic cells" with features suggestive of monocytic/histiocytic origin, possibly Langerhans cell histiocytosis (LCH) was rendered. Following FNA, the patient underwent an incisional thyroid biopsy that confirmed the cytological impression of LCH. In light of the new diagnosis of LCH, the prior suprasellar mass biopsy slides were re-reviewed and rare cells suspicious for LCH were observed. Appropriate treatment for systemic LCH was initiated successfully. This case demonstrates that the presence of enlarged and loosely cohesive cells, especially those with irregular nuclear contours, in thyroid FNA specimens should raise suspicion for LCH. The diagnosis of LCH in FNA specimens is challenging. Additional material should be allocated for ancillary studies to confirm the morphological impression. In our case, not only was the thyroid FNA crucial in diagnosing LCH, but instrumental in initiating a thorough diagnostic work-up for multisystem involvement and thus unmasking the true etiology of the patient's suprasellar mass and associated endocrinopathies.
Keywords: Cytology; Cytopathology; Fine needle aspiration; Langerhans cell histiocytosis; Thyroid.