To understand the different thyroid anomalies, it is important to briefly review normal development of this gland. The thyroid is embryologically an offshoot of the primitive alimentary tract, from which it later becomes separated (69-72) (Figs. 7 and 17). During the third to fourth week in utero, a median anlage of epithelium arises from the pharyngeal floor in the region of the foramen cecum of the tongue (i.e., at the junction of the anterior two-thirds and the posterior third of the tongue). The main body of the thyroid, referred to as the median lobe or median thyroid component, follows the descent of the heart and great vessels and moves caudally into the neck from this origin. It divides into an isthmus and two lobes, and by 7 weeks it forms a “shield” over the front of the trachea and thyroid cartilage. It is joined by a pair of lateral thyroid lobes originating from the fourth and fifth branchial pouches (Fig. 7). From these lateral thyroid components, now commonly called the ultimobranchial bodies, C cells (parafollicular cells) enter the thyroid lobes. C cells contain and secrete calcitonin and are the cells that give rise to medullary carcinoma of the thyroid gland. Williams and associates have described cystic structures in the neck near the upper parathyroid glands in cases in which thyroid tissue was totally lingual in location (72a). These cysts contained both cells staining for calcitonin and others staining for thyroglobulin. This study, they believe, offers evidence that the ultimobranchial body contributes both C cells and follicular cells to the thyroid gland of humans.

As the gland moves downward, it leaves behind a trace of epithelial cells known as the thyroglossal tract. From this structure both thyroglossal duct cysts and the pyramidal lobe of the thyroid develop. The mature thyroid gland may take on many different configurations depending on the embryologic development of the thyroid and its descent (Fig. 18 upper).