Answer: Thyroglossal duct cyst

Histology: A cystic lesion is noted adjacent to the hyoid bone. It lumen is filled with a mucoid material and numerous histiocytes. Its lining epithelium is no longer present. Its fibrotic wall contains collections of follicles with pink colloid-like material. The cells lining the follicles are uniform and lack cytologic atypia.

Discussion: The thyroglossal duct is an embryologic structure – extending from the base of the tongue to the thyroid gland – that normally atrophies during post fetal life. In some individuals the thyroglossal duct persists. Inflammation and cystic dilatation of this persistent embryologic structure gives rise to the thyroglossal duct cyst.

Clinically, the thyroglossal duct cyst is located along the midline of the anterior neck in close proximity to the hyoid bone. Histologically its epithelial lining is cuboidal to columnar and ciliated, but the lining often undergoes squamous metaplasia or disappears altogether in the setting of inflammation. In about half of all thyroglossal duct cysts, thyroid tissue can be identified in the cyst wall. This represents ectopically displaced thyroid follicles, underscoring the important point that the presence of thyroid tissue outside of the gland is not, by itself, diagnostic of thyroid malignancy.

Cases of malignant transformation of this ectopically displaced thyroid tissue are well documented (usually papillary carcinomas), but this remains a rare event.

Surgical removal is generally curative. Complete surgical removal of a thyroglossal duct cyst requires excision of the tract extending from the foramen cecum to and including the mid-section of the hyoid bone.