Background: Poorly differentiated oxyphilic (Hürthle cell) carcinomas are a more recently described variant of poorly differentiated thyroid carcinoma and are characterized by a prominent Hürthle cell component in a solid or trabecular arrangement. Clinically, poorly differentiated oxyphilic carcinomas behave more aggressively as compared to classic Hürthle cell carcinomas, which have a predominantly follicular pattern. Although the histology of these rare thyroid tumors has been reported in the literature, the cytologic features on fine needle aspiration biopsy have not been described before.
Case: A 73-year-old man with a long history of radioactive iodine and levothyroxine therapy for multinodular goiter presented with a painful, rapidly expanding, 6-cm, left thyroid mass with aggressive radiologic features. Fine needle aspiration biopsy of the mass yielded extremely cellular smears with a dual population of medium-sized follicular cells and numerous Hürthle cells. Subsequent thyroidectomy confirmed the malignant nature of this Hürthle cell-rich tumor, warranting a diagnosis of poorly differentiated oxyphilic (Hürthle cell) thyroid carcinoma.
Conclusion: Poorly differentiated oxyphilic thyroid carcinoma is an aggressive variant of Hürthle cell carcinomas and must enter the differential diagnosis when fine needle aspiration biopsy of a radiologically aggressive thyroid mass yields extremely hypercellular smears with a prominent Hürthle cell component.