Primary mucoepidermoid carcinoma in the thyroid gland. A case report including an ultrastructural and biochemical study.

An extremely rare mucoepidermoid carcinoma of the thyroid gland was found in a 44-year-old woman. By light microscopic examination, the tumor consisted of mucous cells and squamoid cells, and was similar to that occurring in any other organs. By electron microscopic examination, the mucous cells displayed prominent mucous droplets, and showed no evidence of colloid production in their cytoplasms. Abundant tonofilaments in aggregates were observed in the squamoid cells. Biochemically, thyroxine (T4) and triiodothyronine (T3) were not detectable in the tissue of this tumor. These findings indicate that this mucoepidermoid carcinoma does not retain the functional properties of thyroid follicular cells; namely, no evidence was found to substantiate a follicular epithelial origin of this tumor.