Background
Ectopic thyroid tissue is a rare congenital anomaly. It may be defined as thyroid tissue located other than its normal place, anterolaterally from the second to the fourth tracheal cartilage. Ectopic thyroid tissue may occur anywhere along the path of descend of the thyroid, during its passage from the floor of the primitive foregut to its final pre-tracheal position at early stages of thyroid gland embryogenesis [1]. Ectopic functioning thyroid tissue is not commonly reported in the presence of MNG (multinodular goiter).
Case presentation
A 38-year-old female presented with complaints of dyspnea and dysphagia with MNG in front of the neck. She was referred to nuclear medicine department, for 99mTc-pertechnetate scan. Neck scintigraphy revealed multinodular goiter (Figure 1(A)). The patient was euthyroid on hormonal analysis. A Surgeon performed total thyroidectomy due to compressive symptoms. Histopathology showed MNG. On the 28th day after surgery, the patient developed a fairly large swelling in the submental region.
Ultrasonography of the neck showed an ovoid solid nodule, measuring 1.6 × 1.7 cm in submental region. She was again referred for the assessment of the tissue in the newly developed lump by 99mTc-pertechnetate scan, with the suspicion that the lump may have thyroid tissue in it.
Her planar scan (Figure 1(B)) demonstrated a rounded focus of intense radiotracer uptake in the submental region with minimal uptake in the region of thyroid bed. Single photon emission tomography + X-ray computed tomography (SPECT-CT) imaging showed a pertechnetate-avid focus in front of the neck which was localized as soft tissue density lesion, measuring 1.6 × 1.7 cm in the submental region with minimal residual tissue in the thyroid bed (Figure 2). Considering pertechnetate avidity and soft tissue density lesion on noncontrast CT, diagnosis of the ectopic thyroid was made.
At present patient had no complaints of pressure or obstruction in the neck. However, presently the main complaint was swelling in the submental region, increasing gradually in size. As the lesion is photon-avid, one of the treatment options is radioiodine therapy followed by lifelong thyroxin replacement therapy. It would decrease the size of ectopic thyroid swelling by lowering the elevated TSH level as the ectopic tissue is subjected to same goitrogenic stimulation as the normally placed thyroid tissue [2,3].
Hormone production from ectopic thyroid tissue is usually insufficient. High TSH drive caused ectopic thyroid swelling to increase in size and become photon-avid after total thyroidectomy in this particular case [4].
Surgery is usually done if the patient has pressure or obstructive symptoms, or there is suspicion of malignancy in ectopic thyroid tissue.
Review of previous scintigraphic findings revealed an area of minimal radiotracer uptake in the submental region. That was actually masked in the presence of large MNG with markedly nonhomogeneous uptake.
The possibility of ectopic thyroid should be kept in mind and evaluated accordingly even in cases of MNG.
Discussion
Ectopic thyroid is the most common form of thyroid dysgenesis [5]. Its incidence is not known yet, however, studies on necropsy suggest that 7-10% of adults can be asymptomatic carriers of thyroid tissue in thyroglossal duct path [6-8]. Ectopic thyroid tissue mostly occurs in the line of descend from the foramen caecum to the mediastinum. [6-9]. It is relatively less common in lateral positions. [9,10]. Most common location is lingual thyroid, accounting 90% of reported cases [5,9-11]. Other sites rarely involved are mediastinum, lungs, porta-hepatis system, duodenum, esophagus, heart, breasts and intratracheal area [9].
The presence of ectopic thyroid tissue in the midline in the submental region in the background of MNG is rarely reported [12,13]. Ectopic thyroid in the submental region with MNG in anterior cervical location is difficult to diagnose preoperatively, as the ectopic tissue can be masked clinically as well as on scintigraphically due to multiple functioning nodules. In the case under discussion, after 28 days of total thyroidectomy high TSH drive caused the swelling to enlarge gradually and become prominent on scintigraphy. The possibility of a recurrent thyroid nodule is excluded on the basis of clinical history and correlative SPECT / CT imaging which revealed that it is located in the submental region. Thyroid scintigraphy is the gold standard for localization of ectopic thyroid tissue. Radiotracer accumulation in normal thyroid area and ectopic thyroid tissue plays a key role in the diagnosis and treatment [14].
Clinically patient presents with a palpable and painless midline cervical mass. It may be associated with hyper or hypo thyroid functioning status [15]. Diseases affecting the normal thyroid gland can also affect the ectopic thyroid tissue [16,17], but benign or malignant neoplastic conditions that affect the ectopic thyroid tissue are very rare [17,18]. Malignant transformation was reported in less than 1% of ectopic thyroids and include all histologic variants with the exception of medullary carcinoma [19,20]. Most common malignancy arising from ectopic thyroid tissue is papillary thyroid carcinoma [21].
The treatment of ectopic thyroid tissue depends upon the symptoms of the patient and the possibility of malignancy. Surgical treatment should be performed when ectopic thyroid in the neck leads to symptoms, such as dysphagia, dysphonia, and dyspnea; and when malignancy cannot be ruled out [14]. However, the risk of malignant transformation in ectopic thyroid tissue is rarely reported in the literature.
Conclusion
Ectopic thyroid tissue should be considered in the diagnosis of a cervical mass, appearing soon after thyroidectomy and even in cases of multinodular goiter in native location. The most appropriate therapeutic option is the surgical resection and pathologic assessment of the swelling because such lesions may harbor primary cancer or metastases of hidden thyroid cancer.