Embryology of the thyroid gland

During the fourth week of development, the foramen cecum develops as an endodermal thickening in the floor of the primitive pharynx at the junction between the first and second pharyngeal pouches, immediately dorsal to the aortic sac. The medial thyroid primordium derives as a ventral diverticulum at the foramen cecum. During the fourth to seventh week of gestation, this primitive thyroid tissue penetrates the underlying mesenchymal tissue and descends anterior to the hyoid bone and the laryngeal cartilages to reach its final adult pretracheal position. During its descent, it is first spherical, and then enlarges and becomes bilobed as it grows caudally. The proximal portion of the diverticulum (connecting the gland and the foramen cecum) retracts and forms a solid fibrous stalk early in the fifth week. This thyroglossal duct ultimately atrophies, but any portion of it may persist to become the site of a thyroglossal duct cyst. The distal portion of this duct gives rise to the pyramidal lobe and levator superioris thyroideae in adults. The lateral thyroid primordia (from the fourth and fifth pharyngeal pouches) descend to join the central component during the fifth week of gestation.

Calcitonin-secreting parafollicular C cells arise within the ultimobranchial bodies (recognized within the lateral thyroid primordia) from neural crest cells of the fourth pharyngeal pouch. They fuse to the medial thyroid anlage during the fifth week of gestation. These cells are therefore restricted to a zone deep within the middle to upper third of the lateral lobes.

The thyroid primordium initially consists of a solid mass of endodermal cells, which later break up into a network of cords with the invasion of the surrounding mesenchyme. The epithelial cords organize into clusters of cells with a central lumen. Follicles begin to appear at the beginning of the second month, and most follicles have been formed by the end of the fourth prenatal month. Thereafter, additional growth is by enlargement of existing follicles. By the end of the third month the gland begins to function, and is able to concentrate radioiodine and synthesize iodothyronines.

Embryology of the recurrent laryngeal nerve

The recurrent laryngeal nerve is originally associated with the sixth branchial arch. It supplies the region of the future larynx that lies caudal to the fifth pharyngeal pouch. The primordium of the vagus nerve is appreciated by the end of the fifth week, whereas the recurrent branch is apparent by the end of the sixth week. Initially, all of the aortic arches are cranial to the larynx, and so the recurrent nerve passes directly to the larynx. As the pharyngeal pouches disappear and the neck elongates, the aortic arch and associated system of vessels remain in the thorax, whereas the larynx moves craniad in the neck. As a result, the recurrent nerve located lateral to the arches must pass medially under the last arch to reach the larynx. This pathway creates a looplike configuration for which the nerve is named. On the left, the sixth arch persists until birth as the ductus arteriosus, and then regresses to form the ligamentum arteriosum. The recurrent laryngeal nerve must pass under it and reascend to reach the larynx. On the right, because the distal part of the sixth aortic arch degenerates, the right nerve is able to ascend as high as the right subclavian artery (part of the right fourth aortic arch). The recurrent nerves therefore become asymmetric.

The course of the recurrent nerves is determined largely by the pattern of the developing arteries. The most frequent variation occurs when the right subclavian artery arises from the distal aortic arch and passes to the right, posterior to the esophagus. The right recurrent nerve now arises from the vagus nerve at the level of the superior pole of the thyroid gland, and enters the larynx directly without forming a recurrent loop. This nonrecurrent nerve is present in about 0.5% to 1% of the population. The main danger to the nonrecurrent nerve is in mistaking it for the inferior thyroid artery during thyroidectomy, or in including it in ligation of the superior pole.

Another variation involves a reversal of the asymmetry between the two recurrent laryngeal nerves. This reversal occurs when a right-sided aortic arch is present, with a right ligamentum arteriosum, causing the right recurrent nerve to loop around the right-sided arch, whereas the left nerve loops around the left subclavian artery. Rarely, a nonrecurrent left nerve may be present in these patients, if the left sublcavian artery is retroesophageal.

Embryology of the recurrent laryngeal nerve

The recurrent laryngeal nerve is originally associated with the sixth branchial arch. It supplies the region of the future larynx that lies caudal to the fifth pharyngeal pouch. The primordium of the vagus nerve is appreciated by the end of the fifth week, whereas the recurrent branch is apparent by the end of the sixth week. Initially, all of the aortic arches are cranial to the larynx, and so the recurrent nerve passes directly to the larynx. As the pharyngeal pouches disappear and the neck elongates, the aortic arch and associated system of vessels remain in the thorax, whereas the larynx moves craniad in the neck. As a result, the recurrent nerve located lateral to the arches must pass medially under the last arch to reach the larynx. This pathway creates a looplike configuration for which the nerve is named. On the left, the sixth arch persists until birth as the ductus arteriosus, and then regresses to form the ligamentum arteriosum. The recurrent laryngeal nerve must pass under it and reascend to reach the larynx. On the right, because the distal part of the sixth aortic arch degenerates, the right nerve is able to ascend as high as the right subclavian artery (part of the right fourth aortic arch). The recurrent nerves therefore become asymmetric.

The course of the recurrent nerves is determined largely by the pattern of the developing arteries. The most frequent variation occurs when the right subclavian artery arises from the distal aortic arch and passes to the right, posterior to the esophagus. The right recurrent nerve now arises from the vagus nerve at the level of the superior pole of the thyroid gland, and enters the larynx directly without forming a recurrent loop. This nonrecurrent nerve is present in about 0.5% to 1% of the population. The main danger to the nonrecurrent nerve is in mistaking it for the inferior thyroid artery during thyroidectomy, or in including it in ligation of the superior pole.

Another variation involves a reversal of the asymmetry between the two recurrent laryngeal nerves. This reversal occurs when a right-sided aortic arch is present, with a right ligamentum arteriosum, causing the right recurrent nerve to loop around the right-sided arch, whereas the left nerve loops around the left subclavian artery. Rarely, a nonrecurrent left nerve may be present in these patients, if the left sublcavian artery is retroesophageal.

Anatomy of the thyroid gland

The thyroid gland derives its name from its resemblance to a shield (Greek: thyreos, shield; eidos, form). The thyroid gland weighs between 15 and 25 g in adults, and comprises 2 lateral lobes connected by a central isthmus. Each lobe is approximately 4 cm in length, 2 cm in width, and 2 to 3 cm in thickness. The isthmus measures about 2 cm in width, 2 cm in height, and 2 to 6 mm in thickness. The gland lies on the anterolateral aspect of the cervical trachea, with the isthmus related to the second, third, and fourth tracheal rings posteriorly. The superior pole lies lateral to the inferior constrictor muscle, and posterior to the sternothyroid muscle. The inferior pole extends to the levels of the fifth or sixth tracheal ring. Posterolaterally the gland overlaps the carotid sheath and its contents. A pyramidal lobe may be present in about 50% of patients, arising from either lobe or the isthmus, and is directed upward, usually to the left. If present, the levator glandulae thyroideae extends from the isthmus or lateral lobe to the hyoid bone or thyroid cartilage. It may be innervated by a branch of the ansa cervicalis or the superior laryngeal nerve.

The thyroid is enveloped by the layers of the deep cervical fascia, and is covered by the strap muscles anteriorly and the sternocleidomastoid muscle more laterally. The true thyroid capsule is tightly adherent to the gland, and continues into the parenchyma to form fibrous septae separating the gland into lobules. Posteriorly, the middle layer of the deep cervical fascia condenses to form the posterior suspensory ligament of Berry, connecting the lobes of the thyroid to the cricoid cartilage and the first two tracheal rings.

The blood supply to the thyroid gland is derived from two pairs of arteries. The superior thyroid artery which is described as the first branch of the external carotid artery, travels along the inferior constrictor muscle with the superior thyroid vein to supply the upper pole of the gland. In this position, the artery lies superficial to the external branch of the superior laryngeal nerve as it courses to supply the cricothyroid muscle. In 16% of cases, the superior thyroid artery may be a branch of the common carotid artery. After passing deep to the infrahyoid strap muscles, the artery divides into anterior and posterior branches at the level of the superior pole to supply their respective surfaces of the lobe. Before branching at the superior pole, each artery gives off a superior laryngeal artery which travels across the thyrohyoid membrane with the superior laryngeal nerve to enter the larynx, and a cricothyroid artery which lies on the cricothyroid membrane near the lower border of the thyroid cartilage. The larger inferior thyroid artery is a branch of the thyrocervical trunk that arises from the subclavian artery. It courses along the anterior scalene muscle, turns medially behind the common carotid artery to descend on the posterior aspect of the lateral lobes before entering the inferior thyroid pole. In its course behind the common carotid artery, the artery exhibits a variable relationship to the sympathetic chain. This vessel may be absent in 6% of patients. The thyroid ima artery is inconsistently present, and arises from the innominate artery, either subclavian artery, right common carotid artery, internal mammary, or aortic arch to supply the thyroid gland near the midline. It may occasionally replace the inferior thyroid artery as one of the principle vessels supplying the gland. It is more common on the right side. Because of its relation to the anterior aspect of the trachea, the thyroid ima artery is in danger of injury during a tracheotomy.

There is a dense network of connecting vessels within the thyroid capsule, with branches passing into the connective tissue between the lobules to form extensive capillary plexuses around individual follicles. The veins draining the capillary plexuses give rise to the inferior, middle, and superior thyroid veins. These join the internal jugular or innominate veins. The paired inferior thyroid veins lie on the anterior aspect of the trachea and anastomose freely with each other before draining into the innominate veins. They represent a potential source of bleeding during thyroidectomy or tracheotomy.

The lymphatics of the gland generally drain with the veins. Those traveling with the superior and middle thyroid vessels drain into the upper and middle deep cervical chain nodes respectively. Lymphatics draining with the inferior thyroid vessels empty into the lower deep cervical chain nodes and the supraclavicular, pretracheal, and prelaryngeal nodes. In addition, lymphatics have been identified that drain directly into the subclavian vein without traveling through lymph nodes.

The thyroid gland has a predominantly sympathetic innervation, from the superior, middle, and inferior cervical ganglia.