Chapter 1

To treat thyroid disease, it is essential to have a thorough knowledge of its embryology. The thyroid is derived from the primitive pharynx as well as the neural crest with the main body arising from epithelial cells of the endoderm and forming the follicles of the gland. Arising as a diverticulum from the floor of the primitive pharynx, the thyroid transforms into a bilobed structure and descends in the midline of the neck. This tract remains attached to the posterior inferior tongue as the thyroglossal duct, and its distal end may go on to form a pyramidal lobe. This serves as the embryologic basis for the formation of a thyroglossal duct cyst as well as nodules within the pyramidal lobe, and underscores the need to completely excise the thyroglossal tract through the hyoid bone to the level of the foramen cecum when the aforementioned cyst is present. It also requires the surgeon to systematically search for a pyramidal lobe when performing a total thyroidectomy because it is present in 30% to 40% of patients and will be the point of persistent or recurrent disease if not identified at the time of operation.1

The neural crest serves as the basis for the formation of the parafollicular cells (C cells). The C cells, which secrete calcitonin, migrate from the fourth and fifth branchial pouches. The combination of these two branchial pouches leads to the formation of the caudal pharyngeal complex, which serves as the precursor to the lateral thyroid lobes (ultimobranchial bodies). Eventually, the lateral lobes join the main body on each side as they descend from the buccal cavity. The C cells ultimately populate the entire gland. The fusion of the ultimobranchial body and the main thyroid body forms the tubercle of Zuckerkandl and can be seen as a slight nodular thickening at the junction of the superior and middle third on the posterior surface of the gland where the lateral lobes meet the main thyroid body.

During development, the third branchial pouch (source of the thymus) is gradually separated caudally. As the thymus, heart, and great vessels descend, it is drawn toward the superior mediastinum. The thymus dissociates, leaving the thyrothymic ligaments as vestigial remnants of their connection. The track of descent of the thymus and great vessels into the superior mediastinum forms the thyrothymic ligaments. Along this path, thyroid rests are formed when the endoderm from the fourth branchial pouch may be pulled down in the descent of the primitive thymus to form retrosternal thyroid components.2 As with the pyramidal lobe, care must be taken to search for these extensions of thyroid tissue to prevent persistence or recurrence of disease when total thyroidectomy is being performed.

### General

The normal thyroid gland lies caudal to the larynx and encircles the anterior and lateral aspects of the first several rings of the trachea. It normally weighs approximately 20 g and is composed of ...

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