VOJNOSANITETSKI ECTOPIC THYROID TISSUE AS DIFERENTIAL DIAGNOSTIC PROBLEM: A CASE REPORT

Inroduction . Mediastinal ectopic thyroid tissue (ETT) represents a rare entity. Clinically, it can manifest with thyroid gland disfunction or with symptoms and signs caused by compressive effect on the surrounding structures, but in the majority of cases it is an asymptomatic condition and incidental finding. All pathologic processes, including malignancy, that can occur in the orthotopic thyroid gland can also develop in the ETT. Case report. Here we discuss a case of 17-year-old female with incidentaly found mediastinal ETT. Beside ETT, patient had an orthotopic thyroid gland and was euthyroid. During follow up, mild compressive symptoms developed. MRI examination showed non-significant increase of the mediastinal mass volume, but due to its morphological changes, a suspicion of another etiology was raised. A discrepancy between the positive technetium-99m pertechnetate and negative 131 iodine radionuclide imaging of the mediastinal mass was highly suspicious for malignancy. Surgery was performed and the pathologist confirmed that it was a coloid goiter in the mediastinal ETT. Conclusion. Mediastinal ectopic thyroid tissue should be taken into account in the differential diagnosis of mediastinal tumor mass. Increase in size of the mediastinal EET, development of compressive symptoms or suspected malignant alteration requests surgical treatment.


Abstract
Inroduction. Mediastinal ectopic thyroid tissue (ETT) represents a rare entity. Clinically, it can manifest with thyroid gland disfunction or with symptoms and signs caused by compressive effect on the surrounding structures, but in the majority of cases it is an asymptomatic condition and incidental finding. All pathologic processes, including malignancy, that can occur in the orthotopic thyroid gland can also develop in the ETT.
Case report. Here we discuss a case of 17-year-old female with incidentaly found mediastinal ETT. Beside ETT, patient had an orthotopic thyroid gland and was euthyroid.
During follow up, mild compressive symptoms developed. MRI examination showed nonsignificant increase of the mediastinal mass volume, but due to its morphological changes, a suspicion of another etiology was raised. A discrepancy between the positive technetium-99m pertechnetate and negative 131 iodine radionuclide imaging of the mediastinal mass was highly suspicious for malignancy. Surgery was performed and the pathologist confirmed that it was a coloid goiter in the mediastinal ETT. Clinical presentation of the ETT includes both hyper-and hypothyroidism, thyroiditis and symptoms caused by compression effect of the ectopic tissue, but it can also be asymptomatic and therefore an incidental finding.
Beside the tests of the thyroid function, imaging methods such as ultrasound, computued tomography (CT), magnetic resonance imaging (MRI), radionuclide thyroid imaging and biopsy, also have an important role in the diagnostic algorithm of the ETT.
Rare locations, functional and morphologic changes in the ectopic tissue can represent a challenge in differential diagnosis, such as in this case.

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We present a 17-years-old female with an incidental finding of two nodules in the left lobe of the thyroid gland on the neck ultrasound examination performed because of repeated sore throats. Further diagnostic procedures included thyroid gland scintigraphy performed with with technetium-99m pertechnetate scintigraphy (Fig. 1), fine needle aspiration biopsy FNAB of the nodules and computed tomography (CT) of the neck and thorax. Thyroid gland scintigraphy with technetium-99m pertechnetate showed normal radionuclid uptake by the gland and uptake by the ectopic thyroid tissue in the upper mediastinum on the left side. Soft tissue mass just below the left lobe the thyroid gland, at level of the apertura thoracis superior, without compressive effect on the trachea was described on CT scan, suspected to be an ectopic thyroid tissue. Thyroid function was normal, while the additional findings included two hypodense nodules, one in both thyroid gland lobes, diameter less than 10mm. Performed FNAB was consisted with a benign follicular nodule.
One year later, patient was examined by an adult endocrinologist. Ultrasound of neck showed small cysts (less than 5mm) in both thyroid gland lobes while the nodules were same as on the previous examination. Thyroid function was normal. (Table 1)  Repeated scintigraphy with 131 I showed normal image of the thyroid gland but this time, there was no uptake by the soft tissue mass in the mediastinum. Patient underwent left sided cervicotomy in general anesthesia, and the surgeon removed the whole mass that was not attached to the thyroid gland. Pathologist confirmed that it was colloid goiter in the ectopic thyroid gland tissue. (Fig. 4) After the surgery, thyroid function remained normal (Table 1.) and the patient was asymptomatic.

Discussion
EET is a rare developmental anomaly. There are studies that suggests genetic base of this anomaly, due to mutation of regulatory genes and transcriptional factors that determine the development of the thyroid gland. Several mutations in genes playing a role during thyroid morphogenesis such as NKX2-1, PAX8, FOXE1,NKX2-5 and TSHR, have been reported, 6 but the molecular mechanisms are not yet fully understood. (1,5,6) EET can appear at any time, but most commonly in childhood, adolescence or in menopause. Female to male ratio is about 4:1. (5) The presence of normal thyroid gland in patients with EET is not necessary. All pathological processes that can develop in the normal gland can also develop in the ectopic thyroid tissue. Clinical symptoms are typically related to the size and location as well as thyroid function. However, in most of the time it is asymptomatic and incidental finding. Increase in size of the EET typically correlates with physiological conditions with increased demands for thyroid hormones that is seen during puberty and pregnancy. (1,2) Mediastinal EET is extremely rare entity, to our knowledge only a few cases were reported (5). It represents about 1% of mediastinal tumors, because of that it is necessary to be included in the differential diagnosis of mediastinal masses with lymphomas, thymic Rarely, a patient with normal TSH can have differences in radionuclide thyroid imaging using technetium pertechnetate vs. iodine scan. This false negative iodine scan could be explained by the presence of the nonorganifying thyroid tissue in EET (follicular cells which have access to the iodine pump, but without organification).
In our case, the first scintigraphy, at the time of diagnosis, was performed with technetium-99m pertechnetate and the uptake by the ETT was present, while the second one performed a year later with more sensitive iodine-131 did not show any uptake of the radionuclide.
Because of the MRI finding, heterogeneous morphology of the tissue, and possible alteration, surgical removal was performed and pathologist confirmed that it was a benign lesion.
In summary, although mediastinal ETT is rare it is necessary to be kept in mind in cases of