Primary mucoepidermoid carcinoma (MEC) of the thyroid is very rare, and its origin has not been fully determined. We report a case of MEC, the origin of which was demonstrated by thyroid specific genes expressed in a metastatic lymph node. A 52-year-old male presented with chest pain, weight loss and diffuse goitre. Ultrasonography showed the thyroid to be diffusely enlarged with numerous small calcifications. The tumour was found to be infiltrating the thyroid, lung, lymph nodes and first thoratic vertebra. A variant type of papillary thyroid carcinoma was suspected by fine needle aspiration cytology of the thyroid. An open biopsy specimen from an axillary lymph node revealed the tumour to be composed of three distinct cell types: mucin-producing cells, intermediate cells and a small amount of epidermoid cells with scattered psammoma bodies. Immunohistochemical studies showed the tumour cells to be negative for thyroglobulin and calcitonin, but positive for CEA. To examine the primary origin of the tumour, the expression of thyroid specific genes in the lymph node specimen was examined by RT-PCR. TTF-1, TTF-2, Pax-8, Na-I symporter and thyroid peroxidase mRNA were detected. The presence of these thyroid-specific mRNAs indicates that this MEC originated from thyroid follicular epithelium. This is the first molecular evidence of dedifferentiation from thyroid follicular cells to MEC.
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