an ectopic thyroid gland is defined as thyroid tissue that is not located anterolaterally to the second to fourth tracheal cartilages . anatomically , an ectopic thyroid can be lingual ( at the base of the tongue ) , sublingual ( below the tongue ) , prelaryngeal ( in front of the larynx ) , or can be found at other rare sites . mediastinal ectopic thyroids are very rare , accounting for less than 1% of all cases , but rare mediastinal ectopic thyroid is also important to consider in the differential diagnosis of mediastinal masses . here , we review a rare case of mediastinal ectopic thyroid presenting with a paratracheal mass compressing the superior vena cava but without symptoms , a condition that has not been reported previously in korea . a 65-year - old male presented with a right paratracheal mass , incidentally detected on a computed tomography ( ct ) scan performed during a health examination after a traffic accident ( fig . he had no history of exposure to radiation , and there was no family history of thyroid disease . the results of his laboratory tests were all within normal limits and included a thyroid stimulating hormone level of 0.96 mu / l ( normal range , 0.40 to 4.00 ) , t3 of 1,090 pmol / l ( normal range , 600 to 1,950 ) , and free t4 of 11.2 the titers of serum thyroid autoantibodies were also within normal ranges ; antithyroid peroxidase antibody was < 25 u / ml ( normal range , < 100 ) and antithyroglobulin antibody was < 25 u / ml ( normal range , < 100 ) . a ct scan of the neck ( fig . 1 ) revealed a 4.5  2.9 cm heterogeneously enhanced mass in the right paratracheal area . the mass was located at the intersection of the caudal margin of the left brachiocephalic vein and the trachea , and it compressed the superior vena cava . the thyroid gland was located in the normal position and showed slightly heterogeneous enhancement and no cervical adenopathy . 2 ) and ebus - transbronchial needle aspiration were performed to obtain a tissue sample of the paratracheal mass . during ebus , the large mass was observed as a right upper paratracheal lesion ( 2r lesion ) . the radiologic diagnosis of the mass by ebus was metastatic adenopathy , castleman 's disease , or tuberculous lymphadenitis . recently , he had undergone a coil embolization for an aneurysm in his left posterior communicating artery ; he is now waiting for the ectopic thyroid tissue to be surgically excised after his general condition has stabilized . ectopic thyroid tissue is the result of abnormal gland migration from the foramen caecum to its normal pretracheal position . lingual thyroid tissue accounts for 90% of these abnormalities , and sublingual ectopic tissues are much less frequent . dual ectopic thyroid has been described , with thyroid gland tissue also present in the normal location . the wall of a thyroglossal duct cyst is a common site for ectopic thyroid tissue , and the presence of a solid mass along a thyroglossal duct cyst should raise a suspicion of ectopic thyroid tissue . furthermore , other rare sites , such as the mediastinum , gall bladder , porta hepatis , and duodenum have also been described . biallelic mutations in foxe1 have been shown to result in thyroid ectopy in mice ; however , to date , no mutations in known genes have been associated with human ectopic thyroid tissues . ectopic thyroid tissue in the thorax with no connection to the cervical thyroid gland is very rare . we could find only a small number of cases of mediastinal ectopic thyroid tissues in the literature , and there were no reported cases in korea . most korean cases reported have been lingual or sublingual ( infrahyoid ) ectopic thyroids , although several cases of dual ectopic thyroids , one lateral ectopic thyroid , and two cases of intratracheal thyroids have been reported [ 4 - 6 ] . to our knowledge , this is the first case of mediastinal ectopic thyroid in korea . the mediastinum is a unique anatomic area containing several structures and pluripotent cells that allow for the development of a range of tumors . mediastinal tumors include primary thymic carcinomas , neuroendocrine carcinomas , germ - cell tumors , and lymphomas , as well as neurogenic , endocrine , and mesenchymal tumors . more rarely , primary thyroid tumors ( adenomas or carcinomas ) may occur in the mediastinum without cervical disease . ct or magnetic resonance imaging is mandatory for evaluating the site and size of the lesion . thyroid scintigraphy with i or technetium-99 m is highly sensitive and specific for detecting normal and ectopic thyroid tissues . ebus has been shown to be of increasing importance in the diagnosis of mediastinal masses . most mediastinal ectopic thyroid cases showed normal thyroid follicles , and one case revealed a papillary thyroid cancer . based on the location , ct - guided fine needle aspiration , ebus - transbronchial needle aspiration or surgical excision is chosen to obtain tissues . because of their invasiveness , mediastinal thyroid carcinomas usually present with dyspnea , wheezing , and chest pain , whereas benign lesions are usually discovered incidentally . mediastinal goiter can remain asymptomatic until the structures located in the thoracic inlet are compressed . the most common symptoms are dyspnea , dysphagia , cough , and hoarseness , and , occasionally , some patients present with superior vena cava syndrome . the chief complaints in reported mediastinal ectopic thyroid cases are painful or pulsating retrosternal mass , dyspnea , and cough . our case was found incidentally and was asymptomatic , although the mass compressed the superior vena cava . if this mass had remained undiscovered , the patient might have suffered superior vena cava syndrome . in mediastinal ectopic thyroid cases , both euthyroidism and hypothyroidism are found , regardless of the presence of a normal thyroid gland . in previous korean studies , hypothyroidism has been found in up to two - thirds of patients with ectopic thyroid , and these patients did not have a normal thyroid gland . however , in one case of hypothyroidism there was a normally located thyroid ; thus , a thyroid function test is necessary , irrespective of the existence of a normal thyroid . in the present case , the thyroid gland was located in the normal position , and the thyroid function test showed euthyroidism . surgery for mediastinal goiters should always be considered , even in elderly patients , because of the high risk of tracheal compression and the low morbidity of the surgery . although there is no real consensus regarding the proper management of mediastinal ectopic thyroids , surgical excision must be considered because they can be malignant , and can have mass effects on the surrounding structures . in summary , ectopic thyroid is a rare condition , and its location in the mediastinum is even rarer . although entirely intrathoracic ectopic thyroids are rare , they must be considered in the differential diagnosis of all mediastinal masses . because they have the potential to become malignant and to compress mediastinal structures , surgical excision of mediastinal ectopic thyroids