The Ectopic Thyroid in the Anterior Mediastinum Coexisting with a Normally Positioned Thyroid Gland in Neck

A  Case Report

Abstract

Introduction: Ectopic thyroid gland is a developmental anomaly resulting from abnormal migration of thyroid tissue from its embryologic origin at the base of the tongue to its normal location in the neck. The coexistence of normally located thyroid tissue with an ectopic thyroid in the anterior mediastinum is an exceptionally rare finding. Most patients with ectopic thyroid tissue are asymptomatic; however, when the ectopic tissue enlarges, it may produce compressive symptoms, including tracheal or esophageal compression.

Case presentation: We report the case of a 50-year-old woman who presented with an insidious onset of dry cough for one year which was unresponsive to conventional treatment. Imaging revealed a superior mediastinal mass along with a normally positioned thyroid gland in the neck. Initially based on imaging a provisional diagnosis of germ cell tumor vs lymphoma was made. An ultrasound guided biopsy through sternal notch was performed with histopathological examination confirming benign, enlarged thyroid tissue without evidence of malignancy.

Conclusion: This case underscores the importance of considering ectopic thyroid tissue within the anterior mediastinum in the differential diagnosis of anterior mediastinal masses, which more commonly include thymic tumors, germ cell tumors, and lymphomas.

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Introduction
Ectopic thyroid is a rare but well known condition in which thyroid tissue is located at a site other than its normal anatomical location within the neck. It can be present anywhere along its path of descent from foramen cecum to anterior lower neck which may occur as a result of abnormalities in development or migration from primitive foregut to pre-tracheal region. Rarely, thyroid tissue may be found beyond this path or away from midline within the trachea, mediastinum, heart, or even the abdomen (1, 2). The presence of an ectopic thyroid with a normally located thyroid gland in neck is extremely rare (3, 4)
Although the incidence varies among the cases reported in literature, the most common type is the lingual thyroid located at the base of the tongue (3). Mediastinal thyroid accounts for less than 1% of ectopic thyroid and often co-exists with a normal orthotopic thyroid tissue (5,6). 
The condition is mostly asymptomatic and is often diagnosed incidentally when a person is evaluated for some unrelated condition. Not infrequently, it may give rise to symptoms based on its location secondary to mass effect, or symptoms related to hypothyroidism and rarely, hyperthyroidism.
We report a case of a female patient with an ectopic thyroid co-existing with a normally located thyroid where the diagnosis was established on histopathology.

Case presentation
A 50-year old female patient, with otherwise no co-morbidity, was referred to us for evaluation of chronic cough that she was experiencing on and off from past one year. The cough was non-productive and mild initially. She had sought medical consultation couple of times but was never evaluated for the same. A respiratory, cardiovascular and ENT examination was done by the treating physician which was unremarkable. Because of persistence of symptoms, a chest X-ray was ordered that revealed an abnormal widening of superior mediastinum with sharply demarcated margins (Fig. 1). The right and left para-tracheal stripes are significantly widened. No other abnormality was found. 
 
Baseline lab investigations were undertaken that revealed normal blood count, and renal and liver function tests. Serum C-reactive protein and ESR were within normal limits. Thyroid function test revealed a TSH level of 1.4mIU/L.

A contrast enhanced CT scan of neck and chest was ordered which revealed a well-defined 6.8x 4.6 cm rounded mass within the superior mediastinum with heterogeneous enhancement pattern. The mass was separate from the thyroid gland. No definite communication was found between the mass and orthotropic thyroid (Fig 2). 

Further evaluation on ultrasound using a high frequency linear transducer showed a well circumscribed fairly homogenous mass lesion in retrosternal location with vascularity upon Doppler analysis (Fig 3). Thyroid gland was seen at its normal location.
A preliminary radiological diagnosis of germ cell tumor vs. lymphoma was made and patient was advised biopsy of the lesion. An ultrasound guided biopsy of the lesion was performed using 18G biopsy gun with co-axial assembly. Histopathological analysis revealed normal thyroid follicles filled with colloid with maintained basal polarity of nuclei (H&E stain) (Fig 4, 5). No signs of malignancy were seen.

Discussion
Ectopic thyroid tissue can be present anywhere along its path of descent from the floor of tongue to pre-tracheal location in lower neck. Rarely, it may be seen beyond this course anywhere from neck up to abdominal cavity or away from midline within salivary glands. It is not uncommon to find an ectopic thyroid tissue with a normally positioned orthotopic thyroid gland. Increase in size in times of physiological stress which may or may not be associated with hyper-or hypothyroidism may lead to mass effect producing symptoms based on its location. Mediastinal thyroid with a normal thyroid in neck is exceedingly rare and patients may present with cough, stridor, dyspnea, dysphagia or signs of thyroid dysfunction. Careful analysis of history and physical examination along with basic laboratory investigations are important. Ultrasound helps in confirming the diagnosis of normal neck thyroid. It also aids in detection of retrosternal extension of thyroid from a purely mediastinal ectopic thyroid. Ultrasound is also useful in guiding biopsy if the mass is visible through suprasternal notch.  Contrast enhanced CT aids in establishing attenuation and contrast characteristics of normal and ectopic thyroid. MRI is another excellent modality to assess similarities between normal and ectopic thyroid. Scintigraphy is also very helpful in localizing ectopic thyroid, however, can be negative incases of necrosis or malignant transformation (7).  Despite all these radiological modalities, the definitive diagnosis is often established only after histopathological analysis of biopsied tissue. Despite most patients being euthyroid, surgical intervention in case of ectopic thyroid may be required especially when symptoms of airway compression exist and rarely in cases of malignant transformation (8).

Conclusion
Mediastinal ectopic thyroid is a rare form among the various ectopic locations of thyroid especially when a normal orthotopic gland coexists. This case underscores the importance of considering ectopic thyroid tissue within the anterior mediastinum in the differential diagnosis of anterior mediastinal masses, which more commonly include thymic tumors, germ cell tumors, and lymphomas. Our aim was to highlight the fact that despite advancement in the radiological methods, definitive diagnosis may still depend on the analysis of cellular structure and any mediastinal mass should always point towards the possibility of the mass being ectopic thyroid tissue.