2 Developmental and Surgical Anatomy of the Thyroid Compartment
2.1 Introduction
Central neck surgery can be complicated by congenital anomalies, neoplastic conditions, and ectopia. A fundamental understanding of the embryological development of structures within the thyroid compartment is paramount for performing safe central neck surgery, and surgeons who operate in this region should be facile in exercising applied embryology to the benefit of their patients.
2.2 Embryology of the Thyroid Gland
The thyroid gland, which is the first endocrine organ in the body to form, begins its development during the third week of gestation. It results from the fusion of two structures: the medial and lateral thyroid anlages. The medial thyroid anlage is derived from the primitive pharynx. The lateral thyroid anlages arise from neural crest cells that form embryonic transient structures called the ultimobranchial bodies. Parafollicular cells (C cells) also originate from the paired ultimobranchial bodies. 1 , 2 , 3 , 4
The medial thyroid anlage begins its formation from a diverticulum (either single or paired), which forms immediately caudal to the primordium of the tongue (tuberculum impar). Development of this structure occurs between the second and third pharyngeal arch at the foramen cecum and originates from an invagination of epithelial cells from the floor of the primitive pharynx (Fig. 2.1). After this initial invagination, the medial thyroid anlage takes form and begins its descent by penetrating the underlying mesoderm during the fifth to seventh week of gestation. At about the same time of descent, the hyoid bone, derived from the first pharyngeal pouch, begins to take form from the condensation and chondrification of the mesoderm. Of particular importance is the route of the medial thyroid anlage migration and its relationship to surrounding structures (Fig. 2.2). The medial thyroid anlage begins posteriorly and most commonly continues anterior to the hyoid and primordial pharyngeal gut. During the migration, the medial thyroid anlage remains attached to the base of the tongue by a continuous lumen, the thyroglossal duct. 5 As the embryo enlongates and undergoes differentiation, this original diverticulum breaks in two, and the proximal part retracts and disappears. By the seventh week of gestation, the primitive thyroid gland reaches its final destination anterior to the trachea between the second and fifth tracheal ring. The thyroglossal duct loses its lumen by the eighth week as the thyroid reaches its normal position. 2 , 3 , 6
Although the medial thyroid anlage contributes to the bulk of the thyroid gland, the paired lateral thyroid anlages (formed from the ultimobranchial bodies) contribute a small but important portion of the final thyroid gland. During the fifth week of development, at the level between the fourth and fifth pharyngeal pouches, cells detach from the pharyngeal wall and fuse with the posterior aspect of the main body of the thyroid as it descends into the neck. These cells originate from neural crest cells and differentiate into the parafollicular cells (C cells), which secrete calcitonin. 2 Ultimately, the fusion between the medial thyroid anlage and the paired lateral thyroid anlages produces thickenings of the gland known as the tubercles of Zuckerkandl. The tubercles of Zuckerkandl serve as consistent landmarks for the recurrent laryngeal nerve (RLN), which mostly runs medial and deep to the tubercles on both sides. 7
Thyroid follicles begin to form at 8 weeks of gestation from epithelial plates. After initial formation, growth continues by budding or division of primary follicles. By the fourth month, follicles can be seen at various stages of development, and during this time they experience the greatest growth in number. Colloid is visible during the 12th week of gestation. Iodide is incorporated into the follicles to mark the beginning of thyroid hormone synthesis, which is then secreted into the fetal circulatory system. 3 , 6
The normal adult thyroid gland weighs approximately 10 to 20 g and is typically located anterior to the trachea, midway between the thyroid cartilage and the suprasternal notch. A fibrous capsule developed from the deep cervical fascia envelopes the gland by dividing into an anterior and a posterior sheath. The thyroid gland is further covered anteriorly by superficial cervical fascia, strap muscles, and the platysma. Laterally, the deep cervical fascia is loosely adherent to the gland. On the posterior portion of the gland, the deep cervical fascia condenses to form a thick suspensory ligament (ligament of Berry), which causes the thyroid to be strongly fixed to the trachea and larynx. In effect, this suspensory ligament facilitates the movement and elevation of the gland as seen during swallowing. The final thyroid gland consists of two lobes, which lie on both sides of the trachea, with a central midline isthmus connecting the lobes. The lobes are typically 4 cm from superior to inferior, 2 cm in width, and 2 to 4 cm in thickness. The isthmus usually measures 1.25 cm. Thyroid disease will often significantly change these dimensions. A pyramidal lobe is present in about 50% of people and is usually connected to the isthmus or one of the lobes. 4 , 8
2.3 Congenital Thyroid Anomalies
Ectopic thyroid tissue is functioning thyroid tissue that exists at any point along the path of migration other than the typical location anterior to the trachea. The majority of aberrant thyroid tissue resides in the midline of the neck along the thyroglossal tract. Lateral neck ectopia is rare and is thought to be derived from abnormal lateral thyroid anlage migration. Ectopic thyroid tissue is reported to occur in 1 of 100,000 to 300,000 people without thyroid disease, and in approximately 1 of 4,000 to 8,000 patients with thyroid disease. Females have a higher predilection for thyroid ectopia than males. 9 , 10
2.3.1 Lingual Thyroid
The lingual thyroid is relatively rare and, because the majority of cases are asymptomatic, the true incidence is not known. Although rare, this entity accounts for 90% of all ectopic thyroid tissue. 9 Lingual thyroid results when the medial thyroid anlage fails to migrate inferiorly into the neck. In 70% of cases, a lingual thyroid is associated with the absence of the normal cervical thyroid. If both locations have thyroid tissue present, usually the lingual thyroid is the only functional tissue. Most patients present with an asymptomatic mass at the back of the tongue, and the diagnosis is confirmed with a radioisotope scan showing increased uptake in the foramen cecum. 4 , 8 , 9 , 10 The lingual thyroid is usually associated with hypothyroidism given that functional ectopic thyroid tissue is usually unable to meet the physiological demands of the body. As a result, the lingual thyroid can hypertrophy from thyroid-stimulating hormone stimulation and become symptomatic, provoking dysphagia, dysphonia, dyspnea, or a sensation of choking. 5 Adults can experience sleep apnea and respiratory difficulty, and in children the presence of a large base of tongue lingual thyroid can cause upper airway obstruction requiring surgical removal. In general, though, asymptomatic lingual thyroid tissue can be left alone without the need for surgical intervention. The lingual thyroid has a low incidence of malignant conversion, with papillary thyroid carcinoma being the most common to develop. 11
2.3.2 Thyroglossal Duct Remnant
The thyroglossal duct usually obliterates by the eighth week of development, but remnants of this attenuated lumen can persist into adulthood. 12 These remnants can present as a cyst or a sinus tract that can develop anywhere along the course of the descent of the thyroid anlage. Anomalies of the thyroglossal duct are the most common congenital neck masses in children.
The most common presenting symptom is a nontender palpable neck mass. 13 The lesions usually appear in the midline or just off the midline between the hyoid bone and the isthmus of the thyroid and measure approximately 2.5 to 3 cm. 12 A thyroglossal duct cyst may not be apparent until it becomes infected or spontaneously ruptures, usually after an upper respiratory tract infection. There does not appear to be a gender predilection, and most patients will present before age 20. 12 Because the thyroglossal duct cyst is usually firmly fixed to the hyoid bone and tongue musculature, swallowing will classically raise the neck mass in a predictable fashion.
Historically, thyroglossal duct cysts were treated with incision and drainage. Recurrence rates with this method of treatment were > 50%. 13 Currently, the standard treatment for a thyroglossal duct cyst is the Sistrunk procedure, in which the cyst is meticulously dissected out along with the central portion of the hyoid bone, where the cyst is anchored, and a cuff of tissue at the base of the tongue. This extensive dissection reduces the recurrence rate to around 5%. 13 Postoperative infections are associated with higher recurrence rates. 13 Thyroid carcinoma can develop within a cyst, especially if the individual has received low-dose irradiation to the head and neck region in the past. Thus all specimens should undergo histological examination.
2.4 Vascular Supply of the Thyroid
2.4.1 Thyroid Gland Vasculature
The thyroid is an extremely vascular gland. Its main arterial blood supply comes from the paired superior and inferior thyroid arteries. The origin of the superior thyroid artery (STA) is variable but most commonly will arise from the external carotid artery. 4 , 14 It can also arise from the bifurcation of the common carotid artery and internal carotid artery, although this is seen less frequently. 14 The STA travels along the external surface of the inferior constrictor muscle and enters into the thyroid gland posteromedially near the upper pole of the lobe. 4 , 15 The artery may branch prior to entering the thyroid gland. Gupta et al proposed a simple classification of STA branching patterns: type I (two major branches), type II (trifurcation of the artery), and type III (no branching following its origin from the common carotid). 14 If the STA trifurcates, these branches may join with vessels on the contralateral side through the isthmus and with the inferior thyroid artery (ITA).
The ITA has a variable distribution and is absent on one side (usually the left) in approximately 0.2 to 6% of cases. 16 After branching from the thyrocervical trunk off the subclavian artery, it ascends posterior to the carotid sheath and has a variable relationship with the sympathetic chain. Prior to entering the gland, the ITA divides into an upper and a lower branch. 15 , 17 The upper branch goes to the posterior aspect of the gland, and the lower branch to the lower pole of the gland. There can be anastomoses with the STA as well as with the contralateral ITA across the midline.
An aberrant ITA (thyroid ima artery) is an inconsistent vessel arising from the aortic arch or the brachiocephalic, right common carotid, or internal thoracic artery. 4 When present, it usually arises on the right side, ascending in front of the trachea.
The thyroid gland is drained by large veins that exhibit extensive cross filling from both sides of the thyroid through a valveless venous plexus (Fig. 2.3). 18
On each side of the gland, three veins arise: the superior, middle, and inferior thyroid veins. Contralateral thyroid veins join through the connective tissue of the lobules of the gland and anastomose behind the capsule. The superior thyroid vein, which drains the upper two-thirds of the ipsilateral thyroid lobe, accompanies its corresponding artery and emerges from the upper pole of the thyroid. It primarily empties into the internal jugular; however, in some cases the superior thyroid vein will terminate into the linguofacial trunk and common facial vein. 19
The middle thyroid vein is present in approximately 38% of dissected lobes and in 62% of patients. 20 In their prospective study of 394 consecutively dissected thyroid lobes Dionigi et al found that the middle thyroid vein is more likely to be present in patients with hyperthyroidism and large goiters. 20 The authors advise that positive identification of the middle thyroid veins in these particular situations can prevent inadvertent injury and excessive bleeding. Like the superior thyroid vein, the middle thyroid vein typically drains the superior two-thirds of the thyroid into the internal jugular vein (a few centimeters below the superior thyroid vein) after crossing the common carotid artery anteriorly. 18 The middle thyroid vein is more commonly found on the right side. 20
The paired inferior thyroid veins emerge from the lower portion of the gland and have abundant cross filling with the contralateral vein. Krausen wrote that these veins are the “ultimate guardians of the trachea” because this plexus of veins in the pretracheal fat is frequently the source of bleeding during and after anterior neck surgeries. 21 There can be a variable number of branches (at least one and up to five) from the inferior thyroid vein, and these branches can terminate into the left brachiocephalic vein, the right brachiocephalic vein, or both vessels. 19 , 21 These veins can form a confluence to drain into the thyroid ima vein that ultimately will flow into the innominate vein. 21
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
