Langerhans cell histiocytosis is a rare, proliferative monoclonal histiocytic disease of unknown cause. Primary involvement of the thyroid gland by LCH is very rare, and most cases show evidence of LCH involving other organs. Herein, we report on a case of thyroid LCH in a patient diagnosed as pituitary gland LCH. When a patient with a history of LCH presents with enlargement of the thyroid gland or a neck mass, and ultrasonography shows a well-defined low echoic mass, LCH should be included in differential diagnosis. Fine needle aspiration can be useful for initial diagnosis; however, for differentiation with thyroiditis or thyroid cancer, core biopsy, positive S-100 protein, and CD1a immunohistochemical staining are needed.