A 66-year-old, non-smoking woman, with a history of sub-total thyroidectomy 33 years ago due to a thyroid nodule. Consultation with a 7-month moderate dyspnea associated with dry cough in the last 3 months. At admission, vital signs were normal; she had no retractions, cyanosis, or stridor. No evidence of neck masses, or jugular engorgement, no edema or collateral circulation, normal auscultation, and normal abdominal physical examination. Chest x-ray showed an 11 cm mass in the upper right hemithorax with regular edges. In the chest CT scan, the heterogeneous rounded lesion showed calcifications inside, 11 cm in diameter, which compressed the superior vena cava without infiltration, the right subclavian vein and displaced the trachea to the left (). Due to the size of the lesion, it was very difficult to differentiate whether it was of mediastinal origin or one of intrapulmonary behavior. In addition, a thyroid gamma scan was performed showing infrasternal uptake suggestive of supernumerary thyroid glands. A CT-guided biopsy was performed from which four fragments were obtained. H&E staining showed no pathological alteration. Immunohistochemistry showed positive TTF-1 which confirms thyroid gland origin. After a multidisciplinary assessment, a resection of the mass via bilateral thoracotomy was decided, in which the mediastinum and right hemithorax were exposed. Intraoperative findings showed a 16 cm mass of cystic content with large desmoplastic reaction, from the mediastinum which displaced the lung and great vessels but did not infiltrate them. The final histopathological diagnosis was of thyroid tissue with findings of multinodular goiter, without signs of malignancy, with cystic dilations containing colloid material, chronic inflammation and presence of foamy histiocytes (). The patient had adequate postoperative clinical evolution.