Thyroid follicular epithelial cell-derived carcinomas: an overview of the pathology of primary and recurrent disease.

The majority of recurrent thyroid carcinomas are histologically differentiated and readily diagnosed by light microscopy or immunohistochemical staining. Infrequently, recurrent carcinoma may be less differentiated than the index lesion, creating diagnostic challenges. Complicating issues include thyroid carcinomas that recur years after initial diagnosis or without clinical elevation of serum thyroglobulin. With a favorable outcome and lengthy survival rates after initial diagnosis of differentiated thyroid carcinoma, there is a greater chance for these carcinomas to recur and as less differentiated carcinoma. As more advanced treatment techniques enter daily practice, the pathology of recurrent thyroid carcinomas will be better defined and more readily diagnosed.