INTRODUCTION

Thyroglossal ducts cysts (TGDC) are the most common form of congenital cyst in the neck. They are cysts of epithelial remnants of the thyroglossal tract and present characteristically as a midline neck mass at the level of the thyrohyoid membrane, closely associated with the hyoid bone. Although most patients with TGDC are children or adolescents, up to one-third are aged 20 years or older [1,2]. Males and females are equally affected.

Patients with a TGDC often have ectopic thyroid glands. (See 'Thyroid ectopia' below.)

EMBRYOLOGY

The anlage of the thyroid gland forms at the foramen cecum of the tongue, which is located on the dorsum of the tongue posteriorly at the apex of the V-shaped sulcus formed by the circumvallate papillae [2]. During the fourth week of gestation, a ventral diverticulum of the foramen cecum is formed from the first and second pharyngeal pouches (the medial thyroid anlage). This diverticulum, with its narrow neck connected to the tongue, descends in the midline of the neck as the thyroglossal tract to the position of the normal thyroid in the base of the neck, where the thyroid lobes separate, by the seventh week. The path of descent is usually anterior to the hyoid bone, but may be posterior to or through the bone, and ends on the anterior surface of the first few tracheal rings.

The tract usually atrophies and disappears by the tenth week of gestation. Portions of the tract and remnants of thyroid tissue associated with it may persist at any point between the tongue and the thyroid (figure 1).

The pyramidal lobe can be thought of as the most caudal remnant of this tract and is present in approximately one-third of normal subjects. The lateral thyroid anlage, consisting of the C-cell precursors, which arises from the neural crest portion of the fourth pharyngeal pouch, ultimately fuses with the descended medial anlage [3]. The pyramidal lobe usually arises from the isthmus of the thyroid, but may arise from the medial aspect of one of the thyroid lobes or both lobes.