Thyroid anlagen begin as an epithelial thickening of endodermal origin during the fourth intrauterine week. This thickening is located in the floor of pharyngeal arch II, between the tuberculum impar (pharyngeal arch I) and copula (arches II through IV) that participate in the formation of the tongue. The anlage rapidly evaginates, coming into contact with the aortic sac of the developing heart. Owing to differential growth, the thyroid migrates from its point of origin, marked by the foramen cecum of the mature tongue (at the junction of the anterior two thirds and posterior one third), to its definitive location. During
this migration, the gland is connected to the tongue by the thyroglossal duct (Fig. 5.1B). The path of migration passes anterior to the developing hyoid bone, whose paired anlagen, from pharyngeal arch II, fuse in the ventral midline and also undergo some rotation. Because of the relationship of the thyroglossal duct to the developing hyoid bone, the duct can be drawn posterocranially with respect to the hyoid, be enveloped in hyoid periosteum or hyoid bone proper, or pass posterior to the hyoid. Typically, the duct degenerates, leaving a short diverticulum at the foramen cecum proximally, a longer cord distally that develops into the pyramidal lobe of the thyroid gland (typically displaced slightly to the left of the median plane), and an intervening fibrous cord. If the discontinuous epithelial cells present in the fibrous remnant differentiate and subsequently assume a secretory function, a thyroglossal duct cyst results.

A thyroglossal duct cyst should be suspected in any person presenting with a median or paramedian lump in the neck, especially if the lump is superior to the level of the cricoid cartilage and if it moves with the excursion of the hyoid bone during swallowing or tongue protrusion (Fig. 5.1C). A lingual thyroid, usually the result of maldescent of the thyroid, has to be considered if the lump is located intralingually. In this case, preoperative evaluation with a radioisotope scan is essential because 65% to 75% of patients with this condition lack other thyroid tissue.