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
 mediastinal ectopic thyroid is a very rare condition , with few reported cases in the literature and no reported cases in korea . 
 this report describes an asymptomatic 65-year - old man with a right paratracheal mass compressing the superior vena . 
 additionally , the epidemiology , clinical manifestation , diagnosis , and management of mediastinal ectopic thyroids are discussed . 
 a mediastinal ectopic thyroid should be considered in the differential diagnosis of all mediastinal masses . 
 surgical excision is recommended for both the diagnosis and treatment of this condition , because of its potential for malignancy and compression of mediastinal structures . 
 this case demonstrates the clinical importance of mediastinal etopic thyroid . 
INTRODUCTION
CASE REPORT
DISCUSSION