Langerhans' cell histiocytosis with thyroid involvement masquerading as thyroid carcinoma

A 28-year-old woman was admitted to our Hospital with a chief complaint ofprogressive gingival swelling and loosening of teeth over about a year.According to past history, she had received total thyroidectomy 2 yearspreviously due to thyromegaly. The thyroidectomy specimen was at firstinterpreted as 'poorly differentiated carcinoma of the thyroid'. One yearago, she began to be aware of gingival swelling and loosening of teeth. Agum biopsy was taken and the pathologic features were similar to her'thyroid carcinoma'. Subsequent investigations, includingimmunohistochemical stain, showed the gum was heavily infiltrated withhistiocyte-like Langerhans' cells which were positive for S-100 protein.Ultrastructural examination of the cells under electron microscope revealedmany typical intra-cytoplasmic Birbeck granules. Langerhans' cellhistiocytosis was diagnosed. Langerhans' cell histiocytosis with thyroidinvolvement is extremely rare and may run a relatively indolent course.Even on a retrospective examination, it may easily be confused with poorlydifferentiated carcinoma of the thyroid. We suspect that this error mayhave been made on other occasions and that the occurrence of this conditionmay be underreported.