Autonomously functioning nodule arising from accessory mediastinal thyroid tissue

INTRODUCTION

Functional imaging using radionuclide Tc99m pertechnetate is routinely used in the evaluation of the problems related to the thyroid gland. Scintiscan has been useful in the visualization of thyroid tissue in ectopic sites due to congenital developmental anomalies. Similarly, the scan helps in establishing the diagnosis of autonomously functioning nodule when the nodule being evaluated appears “hot” with suppression of rest of the gland activity. Here, we present a very rare case of autonomously functioning thyroid adenoma in an ectopic focus of accessory thyroid tissue in the mediastinum.

CASE REPORT

A 30-year-old female patient was investigated for radiating neck pain. Magnetic resonance imaging (MRI) revealed a right paratracheal mass in the mediastinum [Figure 1]. Computed tomography (CT) guided fine needle aspiration cytology (FNAC) from the mass reported it as thyroid adenoma. Clinically, the thyroid gland was not enlarged and serum thyroid hormone levels were within normal limits. Tc99m pertechnetate scan revealed high activity in the mass and poor visualization of thyroid gland in the neck [Figure 2]. Autonomously functioning thyroid adenoma arising from an accessory mediastinal focus with suppression of normal gland in the neck was suspected. Surgical excision of the mass was performed and histologically found to be thyroid tissue with goitrous changes. Tc99m Pertechnetate scan performed 6 weeks after surgery showed restoration of normal activity in the native thyroid gland in the neck with disappearance of the ectopic activity [Figure 3].

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Figure 1 Transaxial MR image of thorax indicates the presence of a mass in the right paratracheal region of the mediastinum

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Figure 2 Tc99m pertechnetate scan of anterior neck and chest shows intense activity in a well-defined vertically ovoid mass lesion in the right paratracheal region within the chest, whereas the thyroid gland is faintly visualized in the normal pretracheal region of the neck

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Figure 3 Tc99m pertechnetate scan repeated after surgery shows disappearance of the ectopic focus of activity in the mediastinum and restoration of normal activity in the thyroid gland in the neck; the gland looks normal. This confirms that the thyroid gland was suppressed earlier due to the presence of autonomously functioning (hot) nodule which in this case was in an accessory focus

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DISCUSSION

Ectopic thyroid tissue may be located anywhere from the base of tongue to diaphragm, sometimes at multiple sites along the thyroglossal tract. Such ectopia above the level of normal thyroid location is usually characterized by absence of total or partial thyroid gland tissue in the normal pretracheal location.[1]

Presence of benign thyroid tissue at lower levels in the mediastinum occurs in three different conditions. Retrosternal extension of goitrous thyroid tissue from the neck is more common and usually with preserved vascular connection to the thyroid in the neck and is referred to as “secondary intrathoracic goiter”.[2] Presence of a thyroid adenoma within the mediastinum co-existing with a multinodular goiter in the neck is referred to as a “migrating nodule”.[3] True ectopic mediastinal thyroid is a distinct entity which occurs as a result of abnormal embryologic migration of thyroid anlage along with the aortic sac. Such developmental aberration which leads to presence of accessory functioning thyroid tissue in the mediastinum other than the cervical thyroid gland is relatively rare. Though the possibility of the mediastinal focus of thyroid tissue being the only functioning gland without any gland tissue in the neck is mentioned in the literature, no such case has been documented so far. Goiter arising from mediastinal thyroid is referred to as “primary intrathoracic goiter”.[4] This condition is responsible for less than 3% of mediastinal masses and the blood supply is usually from local vessels. Patients are usually asymptomatic and the correct diagnosis regarding the mediastinal mass is seldom made preoperatively.[5] Pressure symptoms are rare and there is one case in the literature presenting with hemorrhage and hemoptysis.[6]

There are few case reports in the past where radioiodine neck scan was performed postoperatively (i.e. after surgical removal of the mediastinal thyroid tissue) and documented the presence of normal thyroid gland or some thyroid tissue in the neck, implying the mediastinal thyroid tissue is in fact an accessory focus and not simple ectopia. In case reports where radioiodine scan of the neck and mediastinum was included in the preoperative work up, there is some pre-existing clinically evident abnormality in the cervical thyroid gland.[78] Thyroid ectopia is more often associated with hypofunction and goitrous change following which the patient seeks medical attention. Thyrotoxicosis has been reported in a case of hemiagenesis.[9] Autonomous function in an ectopic or accessory thyroid gland has not been reported so far. Here, we have reported for the first time a case of autonomous function in an accessory mediastinal thyroid suppressing the otherwise normal gland in the neck.