Follicular thyroid carcinoma with clear cell change showing unusual ultrastructural features.

We report a case of follicular thyroid carcinoma with clear cell change displaying unusual ultrastructural features. Light microscopy revealed a metastatic neoplasm in the 12th thoracic vertebra that was composed of nests consisting mainly of clear cells. The tumor was interpreted as a metastasis of a clear cell type of thyroid carcinoma, based on positive immunohistochemical staining for thyroglobulin. However, unlike the metastatic bone lesions, the primary tumor in the thyroid consisted mainly of neoplastic follicular cells with lightly eosinophilic cytoplasm. A distinct follicular formation was noted throughout the tumor. In a portion of the tumor, a solid or stratified disposition of the cells was also observed. The primary tumor contained only a few clear cells similar to those seen in the metastatic bone lesions. In addition, we noted a transition between the follicular cells and the clear cells. Thus, this neoplasm was diagnosed as a follicular thyroid carcinoma with clear cell change, but not a specific type of thyroid neoplasm. Electron-microscopic examination revealed that the cells corresponding to clear cells under light microscopy had abundant cytoplasm characterized by varying degrees of dilation of rough endoplasmic reticulum (RER), which might reflect increased synthesis, storage of secretory products, or both.