Ectopic thyroid tissue presenting as a midline neck mass with absent orthotopic thyroid gland

Clinical History

A 16-year-old female patient presented with a painless, midline neck swelling. The swelling had gradually progressed over the past 1 year. The patient was clinically and biochemically euthyroid. The swelling was evaluated by ultrasound examination and subsequent ultrasound-guided fine needle aspiration cytology was performed.

Imaging Findings

Neck ultrasound was performed with high-resolution linear probe. An approximately 4.2 x 3.6 x 3.1 cm well-defined, mixed heterogeneous lesion with multiple internal cystic areas was noted in the midline of the suprahyoid neck. The lesion showed internal vascularity on colour Doppler evaluation. No normal thyroid tissue was seen in an orthotopic location. No significantly enlarged lymph nodes were noted.

Fine needle aspiration cytology was performed which showed thyroid tissue in the lesion with features of a benign follicular nodule.

Discussion

Ectopic thyroid tissue is a rare developmental abnormality involving aberrant embryogenesis of the thyroid gland during its passage from the floor of the primitive foregut to its final pre-tracheal position [1, 2]. Its prevalence is about 1 per 100000–300000, rising to 1 per 4000–8000 with thyroid disease [1, 2]. In 70–90% of cases, it is the only thyroid tissue present [1, 2, 3, 4].

It is frequently found along the course of the thyroglossal duct or laterally in the neck, but can also be seen at distant sites such as in the mediastinum and subdiaphragmatic. Most cases are asymptomatic. Any disease affecting the thyroid gland may also involve the ectopic thyroid, including malignancy. Primary thyroid carcinomas arising from ectopic thyroid tissue are uncommon and have been reported in cases of lingual thyroid, thyroglossal duct cyst, lateral aberrant thyroid tissue, mediastinum, and struma ovarii. Such malignancies are usually diagnosed only after surgical excision of the lesion. Most tumours are papillary carcinomas [5]. We must distinguish between ectopic thyroid and metastatic deposits emerging from an orthotopic gland, as well as other benign or malignant masses.

Scintigraphy, using Tc-99m, I-131, or I-123, is the most important diagnostic tool to detect ectopic thyroid tissue and shows the absence or presence of the thyroid in its normal location. Thyroid scanning can also unmask additional sites of thyroid tissue. It is both sensitive and specific for differentiation of an ectopic thyroid from other causes of midline neck masses [4]. Ultrasonography is the initial imaging study and can determine the location and extent of lesions aiding presurgical evaluation. Fine needle aspiration cytology (FNAC) can confirm the diagnosis of ectopic thyroid. It is also helpful in differentiating benign from malignant disease [3, 4].

For incidentally diagnosed asymptomatic and euthyroid patients, clinical follow-up is recommended to look for enlargement or development of complications. For mildly symptomatic and/or hypothyroid patients, levothyroxine replacement therapy may be effective. In cases of symptomatic disease, surgery is the treatment of choice, followed by radioiodine ablation and levothyroxine suppression therapy in refractory cases [4, 6].

Differential Diagnosis List

Final Diagnosis

Follicular lesion of ectopic thyroid with absent orthotopic thyroid gland

URL:	https://www.eurorad.org/case/13742
DOI:	10.1594/EURORAD/CASE.13742
ISSN:	1563-4086