Primary paraganglioma of the thyroid gland.

We describe the case of a 46-yr-old euthyroid woman, who was submitted to right lobectomy plus isthmusectomy because of a 30 mm large, rapidly growing thyroid nodule. Two cytological examinations of fine needle aspiration biopsy (FNAB) specimens were not diagnostic. Histology showed a neoplasm composed of nests of chief cells, almost completely replacing thyroid parenchyma, infiltrating the capsule and surgical resection margins, and invading perithyroid tissues. Immunohistochemical analysis revealed that the tumor stained positively to chromogranin, synaptophysin, NSE, S-100 protein and tyrosine hydroxylase, whereas no immunoreactivity was detected against cytokeratin, thyroglobulin, TTF-1, calcitonin and CEA. A diagnosis of thyroid paraganglioma (PG) was finally made. No complications developed following operation. Laboratory analysis and imaging study excluded multicentric disease, metastases to neck or extracervical organs, and multiple endocrine neoplasia (MEN). We report this unusual case, underscore its clinical and immunohistochemical features and discuss differential diagnosis.