RESUMEN
Thyroid rest is isolated deposit of normal thyroid tissue arising in the thyrothymic tract below the lower pole of thyroid gland. Malignant transformation of thyroid rest is very rare. We report an extremely rare case of papillary carcinoma arising from thyroid rest in a 56-year-old male. He presented with hoarseness due to vocal cord palsy. Paratracheal mass in the upper mediastinum was identified by the cause of vocal cord palsy on CT. During surgery, we identified that the mass invaded recurrent laryngeal nerve but had no connection to thyroid gland. Histopathologic examination revealed that the mass was primary papillary thyroid carcinoma and there was no evidence of malignancy in thyroid gland. The post-therapeutic I-131 whole body scan detected several focal hot uptake in lung and mediastinum, suggesting distant metastasis. We should have knowledge of developmental variations of thyroid gland such as thyroid rest and its malignant transformation.