Differentiation of Hashimoto's thyroiditis from thyroid neoplasms in fine needle aspirates

In order to refine the cytodiagnostic criteria for distinguishing Hashimoto's thyroiditis from thyroid neoplasms, aspirates from six cases of Hashimoto's thyroiditis, five Hürthle cell neoplasms and one papillary carcinoma associated with Hashimoto's thyroiditis were reevaluated. Distinguishing characteristics were cell arrangements, nuclear chromatin pattern and nucleolar appearance. Hashimoto's thyroiditis was characterized by flat sheets and clusters of epithelial cells with oncocytic changes or occasionally by cohesive tissue fragments with cells well oriented one to the other. Thyroid neoplasms were characterized by loosely cohesive, syncytial-type tissue fragments with crowded overlapping cells poorly oriented one to the other and/or numerous isolated single cells. The nuclear chromatin of Askanazy cells in Hashimoto's thyroiditis was bland and even while that of neoplastic cells was finely granular, coarsely granular or irregularly clumped. Macronucleoli were present in Hürthle cell tumors but not in the Askanazy cells of Hashimoto's thyroiditis. Epithelial cellularity, lymphoid cellularity, cellular polymorphism and nuclear pleomorphism were not useful criteria for making the differential diagnosis between the two conditions. An admixture of epithelial cells and lymphoid cells indicated Hashimoto's thyroiditis but was not helpful in ruling out an associated neoplasm.