Thyroglossal duct pathology and mimics

The thyroglossal duct is a transient epithelial lined midline channel serving as the path of descent of the thyroid primordium from the foramen cecum, located at the junction of the anterior two-thirds and posterior third of the tongue, down to the thyroid cartilage where definitive thyroid formation occurs [1]. The inferior portion of the duct may develop into the pyramidal lobe and the remainder involutes by the 10th week of gestation (Fig. 1).

Although benign anterior neck masses such as thyroglossal duct cysts are often diagnosed clinically, the clinical presentation of infected cysts, thyroglossal duct carcinoma, or other pathologic mimics may be indistinguishable, necessitating diagnostic imaging. In children or adults with low clinical suspicion for tumor, imaging evaluation may begin with ultrasound. However, if there are atypical sonographic features (i.e., solid component or abnormal vascularity) or high clinical suspicion for tumor, CT or MR imaging is recommended to document an orthotopic thyroid gland and to evaluate for and characterize the features and extent of neoplastic processes [2]. In selected cases, diffusion-weighted or dynamic contrast-enhanced MR imaging can be performed in evaluation of vascular malformations, abscess, or suspicious cervical lymph nodes [3].

If a portion of the thyroglossal duct persists, cystic lesions may arise following cycles of infection and/or inflammation as it is lined with secretory epithelium [4]. Cysts can form anywhere along the course of the duct; however, about 65% of cysts occur at the infrahyoid level [5]. The cyst is histologically composed of epithelial lining of squamous or pseudostratified ciliated columnar epithelium with or without ectopic thyroid gland tissue [6]. As secretions and debris accumulate, suprahyoid cysts can enlarge, push through the floor of the mouth and penetrate down into the anterior neck with resultant symptoms leading to clinical presentation. Thyroglossal duct cysts are the most common non-odontogenic cyst in the neck and most common pediatric cystic neck anomaly and are therefore important to recognize [7].

On imaging, suprahyoid thyroglossal duct cysts are generally midline simple or complex cystic structures, while infrahyoid cysts are commonly paramedian in location. CT will show a smooth, thin-walled hypoattenuating mass (Figs. 2 and 3). On MRI, the lesion will be high signal on T2-weighted images, and low to intermediate signal on T1-weighted images, depending on the degree of proteinaceous or hemorrhagic contents (Figs. 2 and 4) [8]. Ultrasound is also sometimes used for evaluation, especially in the pediatric population, which will show a well-circumscribed anechoic to hypoechoic structure with posterior through transmission; there may be some internal debris (Fig. 5) [3].

During development, the thyroglossal duct wraps inferiorly around the hyoid bone; therefore, a cystic lesion in close association within the hyoid can clue one into the diagnosis (Figs. 2 and 6) [5, 9]. On rare occasions, the duct may become trapped and incorporated into the second and third arch components of the hyoid bone. As such, the Sistrunk resection involves removal of the hyoid body, as well as the entire thyroglossal duct tract and a portion of the tongue base to minimize local recurrence (Fig. 7) [10].

Some thyroglossal duct cysts may not appear as simple thin-walled unilocular lesions. Presence of internal high attenuation, internal debris, and septations generally correlates with prior infection. In active or recent infection, patients may complain of tenderness at the site of a rapidly growing neck mass. Subsequent imaging reveals a thick-walled cyst with rim enhancement and inflammatory changes of the surrounding subcutaneous tissues (Fig. 8). Internal contents may vary, with higher complexity reflecting proteinaceous debris, which may be seen in acute or remote infection [11]. In advanced cases, abscess formation can occur (Fig. 9); these will show restricted diffusion on MR imaging if there is doubt on ultrasound or CT [12]. Fistula formation may develop in severe infections with external cyst rupture or recurrence after resection, although congenital fistula in the newborn have been reported related to complete persistence of the thyroglossal tract after birth (7). Acquired fistulas can be distinguished apart from congenital cases based on later age of presentation (late childhood to early adulthood) and focal irregularity; focal enlargement of the tract on fistulography may represent the site of a ruptured cyst or prior resection [13].

Coexisting carcinoma is rare, occurring in less than 1% of patients and usually arises from thyroid remnants entrapped within the cyst during development. Often these carcinomas are incidentally diagnosed on surgical pathology as the initial disease burden may be microscopic with slow growth. Although thyroglossal duct cysts are anomalies commonly occurring in the pediatric population, coexisting carcinoma usually occurs in patients 40 years of age or older [14]. All subtypes of thyroid carcinoma have been described in thyroglossal duct cysts with the exception of medullary carcinoma due to lack of parafollicular cells in the thyroid anlage. The vast majority of cases represent papillary carcinoma, similar to orthotopic thyroid malignancy. Despite the lack of established predisposing factors, radiation therapy is considered a risk factor along with a female predominance.

On imaging, commonly described features include enhancing wall nodularity and calcifications within the thyroglossal duct cyst (Figs. 10 and 11). Calcification is not usually appreciated on MR imaging, requiring a supplementary CT examination [4, 6]. Calcifications are a more specific indicator of malignancy than solid components, as the latter can also be seen in inflammatory processes. Rarely, a purely solid midline or paramidline lesion may be malignant and should be considered when there is associated central FDG avidity or the presence of suspected regional metastatic lymphadenopathy (Fig. 12) [14]. Fine needle aspiration cytology may be obtained of suspicious solid components to confirm the diagnosis [15].

There are many mimics of thyroglossal duct cysts, and it is important to recognize these as each has different clinical implications. Close attention to the age of presentation, location of the lesion, association with surrounding structures, and internal architecture can clue one into the correct diagnosis (Table 1).

As eluded to earlier, thyroid tissue remnants can be found anywhere along the midline tract of the thyroglossal duct, extending from the foramen cecum to the thyroid cartilage (Fig. 19). Ectopic thyroid tissue depositing lateral to the expected midline course is rare [1, 23]. About 90% of reported cases occur at the base of the tongue, and therefore, are called lingual thyroid [18]. Usually asymptomatic and incidentally discovered, a lingual thyroid may come to clinical attention with development of dysphagia and stridor, most commonly in children [24].

A lingual thyroid demonstrates the same imaging characteristics as normal thyroid tissue: high attenuation structure relative to adjacent muscle on CT with diffuse enhancement and high signal on T1-weighted images with variable enhancement on MRI [1, 23]. As expected, lesions of normal thyroid tissue such as nodules can also be seen in lingual thyroid and serve as key diagnostic clues [25]. It is important to closely inspect the thyroid bed in cases of lingual thyroid when surgery is planned for removal, as orthotopic tissue is absent in about 70–80% of cases; a patient with little or no thyroid tissue in the expected bed warrants supplemental thyroid hormone after surgery [26]. Radionuclide technetium 99 m-pertechnetate or radioiodine imaging (utilizing I¹²³ or I¹³¹) can confirm the diagnosis of ectopic thyroid tissue (Fig. 20) [27].