Thyroglossal cyst is a fluid-filled swelling in the midline of the neck usually found at the level of the hyoid bone or inferior to it. It represents the remnant of the thyroglossal duct which develops during embryogenesis in the fetal foramen cecum (tongue base) and then migrates to its final destination in the middle to lower anterior cervical region. The duct is normally obliterated in the first trimester while a persistence of the duct is associated with the formation of the cyst.
Thyroglossal cyst (TGC) is a fluid-filled remnant of the thyroglossal duct which runs from the foramen cecum at the fetal base of the tongue to the lower anterior cervical region   . TGC develops due to the anomalous formation and migration of the thyroid gland during embryogenesis. TGC is the commonest midline cervical mass found in clinical practice . It is usually located at the level of the hyoid bone or inferior to it near the thyrohyoid membrane but can also be found in the lingual or submental region  .
TGC can present as an asymptomatic mass in children, frequently around the age of 6 years. Approximately 7% of TGCs have been reported to occur in adults and the most common presentation in adults is an underlying infection of the cyst  . Clinical features of TGC include a midline cervical mass which moves with swallowing or protrusion of the tongue, sometimes accompanied by symptoms of a sore throat, odynophagia, dysphagia, and hoarseness. Sudden enlargement of the cyst, especially if it is located in the lingual or submental region can lead to airway obstruction. Occasionally TGC may present as an infected cyst or as a malignancy within the cyst   . Cases of papillary thyroid carcinoma, as well as, squamous carcinoma have been reported in older adults   .
TGCs are diagnosed clinically but radiological testing is required to confirm the diagnosis and to detect the presence of the thyroid gland in children. Physical examination finding of a midline cervical mass which moves with swallowing or protrusion of the tongue is the classical presentation of TGC.
A majority of clinicians prefer ultrasound for initial diagnosis, especially in children, as it is non-invasive, easily available, is not associated with exposure to radiation, and does not require sedation. Ultrasound reveals fluctuant, cystic mass which occasionally spread cervical strap muscles. The fluid within the cyst appears anechoic although in some cases the fluid may contain debris. The walls of the cyst are thin and avascular. Rarely, in a presence of an infection, there may be evidence of inflammatory changes.
Computed tomography (CT) and magnetic resonance imaging (MRI) are useful to delineate the extent of the mass and its anatomic relationship to neighboring structures    . CT findings include a well circumscribed cystic lesion with rim enhancement . The cysts have a thin wall and appear smooth, well defined, homogeneously attenuated, and located usually within 2 cm of the midline. There may be displacement of the sternocleidomastoid either posteriorly or posterolaterally and in certain cases, TGCs may be embedded in the infrahyoid (strap) muscles. CT scan is preferred in adults as TGCs are rare in adults and therefore have a broader differential diagnosis and also because the incidence of malignancies associated with TGCs in adults is higher which CT scans are able to detect better than the ultrasound.
MRI is preferred when TGCs are located near the base of the tongue. TGC features on MRI are a low, variable T1 signal in uncomplicated cysts and a high signal if there is previous hemorrhage or infection . T2 image shows a high signal.