Imaging of ectopic thyroid tissue and thyroglossal duct cysts.

When a child or young adult presents with a mass in the anterior portion of the neck, diagnostic considerations include a thyroglossal duct cyst and ectopic thyroid tissue. These entities are often suspected clinically, and imaging provides an opportunity to evaluate the extent, confirm the diagnosis, and evaluate for complications. Imaging characteristics of a thyroglossal duct cyst as a simple cyst and of ectopic thyroid tissue as a hyperattenuating soft-tissue mass can help identify these lesions at computed tomography (CT); however, intrinsic magnetic resonance, CT, and ultrasonographic imaging characteristics alone cannot be used to confirm the diagnosis. Rather, knowledge of the typical course of the thyroid primordium during embryologic development is essential to understand the variant locations along this path where thyroid tissue can be found. The migration of thyroid primordium begins at the foramen cecum at the base of the tongue and then loops around the hyoid bone anteriorly and inferiorly and descends anteriorly to the thyrohyoid membrane into the orthotopic location in the infrahyoid portion of the neck. Thyroid ectopia is categorized into one of four typical locations with respect to this embryologic course: (a) the base of the tongue, (b) adjacent to the hyoid bone, (c) the midline infrahyoid portion of the neck, and, rarely, (d) the lateral part of the neck. The differential diagnosis includes metastatic thyroid carcinoma, branchial cleft cyst, lymphatic malformation, abscess, saccular cyst, epidermoid cyst, and squamous cell carcinoma. The relationship of a mass to landmarks such as the foramen cecum, hyoid bone, strap muscles, thyrohyoid membrane, and thyroid cartilage can help differentiate a thyroglossal duct cyst and ectopic thyroid tissue from other anterior neck masses when the embryologic thyroid course is considered.