Fig. 4

Immunoperoxidase for thyroglobulin showing the solid nests, which are negative while the follicles are positive, including a small trapped microfollicle within the larger nest of metastatic cells. Thyroid transcription factor 1 and calcitonin were equally negative; however, cytokeratin 7 was focally positive and synaptophysin was expressed by the majority of cells. This raises the possibility of a carcinoma with neuroendocrine features