Follicular Adenoma of the Thyroid With Spindle Cell Metaplasia Mimicking Insular Thyroid Carcinoma and Lacking Thyroglobulin Expression

Abstract

Abstract We report a case of a well-encapsulated thyroid adenoma with areas of spindle cell metaplasia mimicking insular or poorly differentiated growth pattern. Emphasis on overall features, especially encapsulation and lack of invasion, is critical in the evaluation of thyroid neoplasms. In this example, there was loss of thyroglobulin expression, which to date has not been reported in spindled metaplasia but was not considered diagnostically relevant in the face of low proliferative activity and lack of mitotic figures.