Embryology and Surgical Anatomy of the Thyroid and Parathyroid Glands

Abstract

The primordial thyroid gland is first identifiable during the fourth week of gestation, beginning as an endodermal invagination of the tongue at the site of the foramen cecum (Fig. 2.1). The foramen cecum lies where the midline intersects the sulcus terminalis, which divides the tongue into anterior two thirds (oral part) and posterior one third (pharyngeal part). The thyroid diverticulum begins its descent through the tongue carrying with it the thyroglossal duct. The path of descent carries the developing gland anterior to the hyoid bone and the larynx. During the descent in the fifth week, the superior part of the duct degenerates. By this time, the gland has achieved its rudimentary shape with two lobes connected by an isthmus. It continues to descend until it reaches the level of the cricoid cartilage at about the seventh week. By the 12th week of development, thyroid hormone is secreted. The distal part of the thyroglossal duct degenerates but may remain as a pyramidal lobe.