Answer: Parasitic thyroid nodule with Hashimoto’s thyroiditis

Histology: The specimen consists of an oval nodule of lymphoid tissue with germinal centers and collections of thyroid follicles. The follicular spaces are lined by follicular epithelium with enlarged and elongated nuclei showing optic clearing, overlapping, and occasional grooves. Papillary formations, psammoma bodies and invasive tumor growth are not apparent.

Discussion: The anatomic separation of a peripheral benign thyroid nodule from its connection to the main gland is sometimes referred to as a parasitic thyroid nodule. Usually this phenomenon represents the focal expression of a hyperplastic nodule in the setting of multinodular hyperplasia. Less common but more problematic is when a parasitic thyroid nodule is encountered in the context of nodular Hashimoto’s thyroiditis. In these instances, the prominent lymphoid component of Hashimoto’s thyroiditis is easily mistaken as a residual lymph node. Concurrently, the follicular epithelium shows the reactive changes of Hashimoto’s thyroiditis including nuclear enlargement with optic clearing. Taken together, these features invite the misinterpretation of metastatic papillary thyroid carcinoma. There are some guidelines that may be useful in avoiding this pitfall. A parasitic thyroid nodule with Hashimoto’s thyroiditis has thyroid follicles distributed throughout the nodule, while metastatic papillary thyroid carcinoma is usually limited to the subcapsular sinuses. Additionally, papillary formations and psammoma bodies are usually present within metastatic papillary carcinoma, while they are invariably absent in a parasitic thyroid nodule with Hashimoto’s thyroiditis.