Answer: Parasitic thyroid nodule with Hashimoto’s thyroiditis

Histology: The specimen was removed as a rounded circumscribed tan nodule measuring 1.5 cm. Microscopically, the nodule is comprised of lymphoid tissue with well-formed germinal centers. Dispersed among the lymphoid tissue are epithelial cells with prominent oxyphilic change, nuclear enlargement and chromatic pallor. The cells form follicles filled with pink colloid secretions.

Discussion: Although thyroid cancer can certain infiltrate soft tissues of the neck and metastasize to regional and distant sites, the presence of thyroid tissue outside of the thyroid gland is not necessarily indicative of a malignant process. Thyroid tissue can be encountered outside of the thyroid gland resulting from abnormal migration during embryogenesis (e.g. midline thyroid ectopia), following trauma or surgical manipulation of the thyroid gland (mechanical implantation), or as a component of a teratoma (struma ovarii). Finally, thyroid nodules sometimes detach from the main thyroid gland and migrate laterally into the neck where they can exist as an autonomous nodule (“parasitic” thyroid nodule). Although these parasitic nodules tend to be quite harmless, they can present a diagnostic pitfall for the unwary pathologist, particularly when they are involved by Hashimoto’s thyroiditis. In these instances, the prominent lymphoid stroma may be quite easily mistaken for a lymph node harboring foci of metastatic thyroid carcinoma. Confusion can be avoided by remembering that the changes present in the presumed lymph node entirely mirror the changes present in the parenchyma of thyroid glands involved by Hashimoto’s thyroiditis, and that these changes include reactive nuclear atypia.