Pap-smear, 400x.   Oxyphilic cells with prominent nucleoli. Several atypical, pleomorphic forms are found. The severity of atypia has little if any relevance in differentiation benign from malignant Hürthle-cell tumors.
This pattern with many polygonal forms is very similar to that seen in medullary cancer. The occurrence of microfollicles is of greatest help in cytological differential diagnostic.