The thyroid gland is the first endocrine gland to develop in the embryo. It is derived from one median and two lateral anlages that fuse and entail a middle descent to reach their final position by the eighth week of gestation. Failure of the normal descent of the thyroid gland results in ectopic thyroid tissue [2]. The prevalence of ectopic thyroid tissue is nearly 1/100,000–300,000. The male/female ratio is 1/4. Ectopic thyroid tissue can be seen in any situation on the migration pathway of the thyroid gland, from the foramen caecum to the mediastinum [3, 4]. It is typically encountered in the midline cervical region. The presence of normal thyroid tissue laterally in the neck has rarely been described [5, 6].
A failure of the lateral anlage to fuse with the median anlage or an aberrant migration with cell rests deposited laterally during the development of the gland can result in the development of lateral aberrant thyroid tissue. Other possible causes include implantation of thyroid tissue during surgery of a normal localized thyroid gland or metastasis of a thyroid carcinoma [7, 8].
The common differential diagnoses of a submandibular mass are lymphadenopathy of various etiologies, submandibular inflammatory or malignant lesions, or tumors of the inferior pole of the parotid gland. Although rare, thyroid ectopia should be considered among the differential diagnoses of a submandibular mass, independent of the submandibular gland, as is the case in our observation [9, 10].
Ectopic thyroids are usually asymptomatic and may become clinically evident with the development of goiters, hyperthyroidism, or malignancy.
Scintigraphy with technetium (Tc 99m) and iodine, in association with ultrasonography and fine needle biopsy (FNB), plays an important role in the diagnosis of ectopic thyroid tissue [11]. Thyroid scintigraphy scanning detects all sites of thyroid tissue as well as hyper-functional parenchyma [12]. FNB, when conclusive, contributes to the preoperative diagnosis and appropriate therapeutic approach. It has high diagnostic accuracy in the differentiation of benign and malignant processes with an increased sensitivity when biopsy is performed under ultrasound guidance [11].
We managed our case as a submandibular mass of unknown etiology. The likelihood of an ectopic thyroid tissue was low in particular with the evidence of an orthotopic thyroid gland on radiological evaluation. Therefore, scintigraphy was not performed. FNB was non-diagnostic so that surgery became essential.
Submandibular ectopic thyroid has to be differentiated from metastatic thyroid cancer. The rate of malignant transformation in ectopic thyroid is comparable to that in normally located thyroid [13].
Ectopic parenchyma may also be dysfunctional. Eli et al. [14] reported a rare case of ectopic submandibular thyroid causing hyperthyroidism in a patient with a submandibular mass and pre-existing thyroid disease that presents with a deteriorating thyroid function of no apparent reason. The resection of the submandibular mass, which proved to be of a thyroid nature, allowed the correction of the thyroid function.
The therapy of choice for ectopic thyroid tissue is its surgical removal. However, before surgery, it is necessary to ensure that other orthotopic thyroid tissues are functional to avoid the risk of iatrogenic hypothyroidism. In fact, in cases of lateral cervical ectopic thyroid with simultaneous eutopic thyroid, which is a rare situation, the ectopic tissue may be the only functional up to 70% [15, 16].
The treatment of ectopic thyroid depends on its size, local symptoms, functional status of the thyroid gland, and complications [15].The surgical indications for an ectopic thyroid tissue include the risk of malignancy, refractory hyperthyroidism, signs of compression, or esthetic deformity [17].
