D. Thyroid paraganglioma
Because of their rarity and overlapping features with other common thyroid entities, thyroid paragangliomas (TPGLs) are rarely suspected on fine-needle aspiration cytology. Although histopathologic assessment can render a definite diagnosis in most cases, immunohistochemistry may be helpful, especially in cytology specimens. In this patient, distinct from the PTCs in the left lobe, isthmus, and lymph node, the right-sided nodule showed an organoid zellballen pattern. The tumor cells showed amphophilic granular cytoplasm, round to oval nuclei, and salt-and-pepper chromatin and were positive for synaptophysin but negative for AE1/AE3, thyroid transcription factor–1 (TTF-1), thyroglobulin, paired-box gene 8, monoclonal carcinoembryonic antigen, and calcitonin. Intact succinate dehydrogenase (SDH) B expression was detected by immunohistochemistry. Focal sustentacular cells were highlighted by an S100 immunostain. These findings confirmed the diagnosis of TPGL.