Follicular carcinoma of the thyroid with extensive clear-cell differentiation: a potential diagnostic pitfall.

Clear-cell features have been recognized in several different thyroid neoplasms. A case of thyroid follicular carcinoma with extensive clear- and Hurthle-cell features is described in a 37-yr-old white female, with cytochemical and immunohistochemical analysis. The tumor of the thyroid gland, with anterior neck soft-tissue extension, displayed clear cells on fine-needle aspiration, which were negative for thyroglobulin. The surgical specimen displayed predominately clear cells (80%), and only the nonclear-cell areas stained for thyroblobulin. Proper categorization of clear-cell lesions of the thyroid and soft tissues requires a multimodality approach, involving clinical/pathological correlation, morphological analysis, and ancillary tissue studies. Immunohistochemical stains for thyroglobulin are quite definitive in making the distinction between primary clear-cell thyroid tumors vs. metastatic clear-cell tumors. Cytologists should be aware, however, that the clear-cell areas of thyroid tumors might not stain for thyroglobulin.