Lying deep to the sternohyoid, sternothyroid, and omohyoid muscles, the thyroid gland is in the visceral compartment of the neck. This compartment also includes the pharynx, trachea, and esophagus and is surrounded by the pretracheal layers of fascia.
The thyroid gland arises as a median outgrowth from the floor of the pharynx near the base of the tongue. The foramen cecum of the tongue indicates the site of origin and the thyroglossal duct marks the path of migration of the thyroid gland to its final adult location. The thyroglossal duct usually disappears early in development, but remnants may persist as a cyst or as a connection to the foramen cecum (i.e., a fistula).
There may also be functional thyroid gland: associated with the tongue (a lingual thyroid), anywhere along the path of migration of the thyroid gland, or extending upward from the gland along the path of the thyroglossal duct (a pyramidal lobe).
Two major arteries supply the thyroid gland.
Superior thyroid artery. The superior thyroid artery is the first branch of the external carotid artery (Fig. 8.179). It descends, passing along the lateral margin of the thyrohyoid muscle, to reach the superior pole of the lateral lobe of the gland where it divides into anterior and posterior glandular branches:
The anterior glandular branch passes along the superior border of the thyroid gland and anastomoses with its twin from the opposite side across the isthmus (Fig. 8.179).
The posterior glandular branch passes to the posterior side of the gland and may anastomose with the inferior thyroid artery (Fig. 8.180).
Inferior thyroid artery. The inferior thyroid artery is a branch of the thyrocervical trunk, which arises from the first part of the subclavian artery (Figs. 8.179 and 8.180). It ascends along the medial edge of the anterior scalene muscle, passes posteriorly to the carotid sheath, and reaches the inferior pole of the lateral lobe of the thyroid gland.
At the thyroid gland the inferior thyroid artery divides into an: inferior branch, which supplies the lower part of the thyroid gland and anastomoses with the posterior branch of the superior thyroid artery, and an ascending branch, which supplies the parathyroid glands.
Occasionally, a small thyroid ima artery arises from the brachiocephalic trunk or the arch of the aorta and ascends on the anterior surface of the trachea to supply the thyroid gland.
Three veins drain the thyroid gland (Fig. 8.179):
The superior thyroid vein primarily drains the area supplied by the superior thyroid artery.
The middle and inferior thyroid veins drain the rest of the thyroid gland.
The superior and middle thyroid veins drain into the internal jugular vein, and the inferior thyroid veins empty into the right and left brachiocephalic veins, respectively.
Lymphatic drainage of the thyroid gland is to nodes beside the trachea (paratracheal nodes) and to deep cervical nodes inferior to the omohyoid muscle along the internal jugular vein.
The thyroid gland is closely related to the recurrent laryngeal nerves. After branching from the vagus nerve [X] and looping around the subclavian artery on the right and the arch of the aorta on the left, the recurrent laryngeal nerves ascend in a groove between the trachea and esophagus (Fig. 8.180). They pass deep to the posteromedial surface of the lateral lobes of the thyroid gland and enter the larynx by passing deep to the lower margin of the inferior constrictor of the pharynx.