Surgical anatomy of the thyroid gland

INTRODUCTION — Emil Theodor Kocher and Theodor Billroth pioneered the surgical management of thyroid disease in the late 1800s. Their surgical techniques, knowledge of thyroid physiology, and antisepsis practices transitioned a life-threatening operation to one with acceptable morbidity. Meticulous technique, combined with an understanding of thyroid embryology and anatomy, is the foundation of the surgical management today. A thorough understanding of thyroid anatomy is central to the performance of safe thyroid surgery [1].

Thyroid anatomy is discussed here. The indications for thyroidectomy and surgical techniques are discussed elsewhere. (See "Surgical management of hyperthyroidism" and "Differentiated thyroid cancer: Surgical treatment" and "Thyroidectomy".)

FUNCTION — Thyroid hormones, thyroxine (T4), and 3,5,3'-triiodothyronine (T3) are critical determinants of brain and somatic development in infants and of metabolic activity in adults; they also affect the function of virtually every organ system. Thyroid hormone biosynthesis and secretion are maintained within narrow limits by a regulatory mechanism that is sensitive to small changes in circulating hormone concentrations.

The processes of thyroid hormone synthesis, transport, and metabolism and the regulation of thyroid secretion and actions of thyroid hormone are discussed elsewhere. (See "Thyroid hormone synthesis and physiology" and "Thyroid hormone action".)

SIZE AND LOCATION — In healthy adults without iodine deficiency, a normal thyroid gland is approximately 4 to 4.8 x 1 to 1.8 x 0.8 to 1.6 cm in size, with a mean sonographic volume of 7 to 10 mL and weight of 10 to 20 grams [2-4]. Thyroid volume measured by ultrasonography (US) is slightly greater in men than women, increases with age and body weight, and decreases with increasing iodine intake [5]. The thyroid is one of the most vascular organs in the body. Thus, US measurements of thyroid volume and even nodule size can differ markedly from the size after devascularization and resection.

The normal thyroid gland is immediately caudal to the larynx and encircles the anterolateral portion of the trachea (figure 1 and figure 2) [1]. The thyroid is bordered by the trachea and esophagus medially and the carotid sheath laterally. The sternocleidomastoid muscle and the three strap muscles (sternohyoid, sternothyroid, and the superior belly of the omohyoid) border the thyroid gland anteriorly and laterally (figure 1). There are many anatomic variations in thyroid gland shape and extent. Conditions such as thyroiditis, malignancy, goiter, substernal goiter, hypothyroidism, prior cervical surgery, and prior radioiodine ablation can significantly distort, enlarge, or shrink the thyroid gland and/or obscure its anatomic borders.

Thyroid lobes — The thyroid has two parts or lobes that are connected by the thyroid isthmus, a narrow band of thyroid tissue. The thyroid lobes extend from the isthmus superiorly to the mid-thyroid cartilage and laterally to the common carotid arteries. The lobes have superior and inferior poles. The thyroid lobes can be flat or globular but always have a three-dimensional shape as they curve around the trachea posteriorly.

Thyroid isthmus — The thyroid isthmus is usually a narrow band of thyroid tissue overlying the second and third tracheal rings and connecting the two lobes of the thyroid. The thyroid isthmus can be wide, long, or even absent and may have a pyramidal lobe.

Pyramidal lobe — The pyramidal lobe extends superiorly from the isthmus and can reach the level of the hyoid bone. A fibrous tract, which is the obliterated thyroglossal duct, extends from the pyramidal lobe to the hyoid bone and may harbor a thyroglossal duct cyst. (See "Thyroglossal duct cyst, thyroglossal duct cyst cancer, and ectopic thyroid".)

The pyramidal lobe can be long, short and stubby, bifid, or absent. A bifid lobe occurs as paired structures just to the right and left of the midline. A cadaver study showed that a pyramidal lobe was found in 55 percent of individuals (32/58) and was found more frequently in men than in women [6].

The identification and resection of a pyramidal lobe is an important step in assuring that a total or near-total thyroidectomy has been completed adequately. Therefore, the anterior cervical region requires careful examination during a total thyroidectomy to avoid leaving residual thyroid tissue.

Tubercle of Zuckerkandl — The tubercle of Zuckerkandl, a pyramidal extension of the thyroid gland, is located on the posterior aspect of each thyroid lobe [7]. The recurrent laryngeal nerve (RLN) usually traverses the posterior aspect of the tubercle, which can help the surgeon find and identify the nerve. The tubercle of Zuckerkandl should be carefully elevated and rotated medially to identify the nerve as it courses posterior to the tubercle.

Occasionally the tubercle tissue passes behind (deep) to the RLN as it enters the larynx, in effect tethering this portion of the thyroid gland to the trachea. The surgeon should look for this anatomic situation and preserve the nerve throughout its course to avoid inadvertent division of the nerve (figure 3) [8].

Ectopic thyroid tissue — Abnormalities in development during embryogenesis may result in ectopic thyroid tissue [9]. Lingual thyroid tissue along the path of the thyroglossal duct is the most common site of thyroid ectopy. (See "Thyroglossal duct cyst, thyroglossal duct cyst cancer, and ectopic thyroid".)

Ectopic thyroid tissue can be benign or malignant [10]. Malignant transformation of ectopic thyroid tissue is rare. However, if thyroid tissue is found in the lateral cervical lymph nodes (lateral aberrant thyroid), a metastasis of a malignant thyroid tumor should be excluded.

EMBRYOLOGY — Thyroid development is detectable in the third week of gestation. The thyroid is primarily derived from the endoderm. The ventral portion of the fourth pharyngeal pouch will develop into the lateral thyroid lobes (figure 4).

The pyramidal lobe, present in up to 55 percent of patients, originates from the migration of the thyroglossal duct that descends from the pharynx at the foramen cecum of the tongue and attaches to the thyroid isthmus [6,11]. The thyroglossal duct is usually obliterated after its descent. If it remains patent, the patient may develop a thyroglossal duct cyst. (See "Thyroglossal duct cyst, thyroglossal duct cyst cancer, and ectopic thyroid".)

The ultimobranchial bodies (transient embryonic structures) consist of neural crest cells from the fourth and fifth branchial pouches, which migrate to the upper third of the thyroid lobes. While the neuroendocrine parafollicular cells (C cells) have long been thought to be derived from the ultimobranchial bodies, some studies have suggested that the C cells may in fact be derived from endoderm; the clinical implications of this are yet unknown [12,13]. The ultimobranchial bodies fuse with the posterior lobes of the thyroid. C cells, which produce calcitonin, make up only approximately 0.1 percent of the thyroid mass and are concentrated in the upper thyroid lobes. Multicentric hyperplasia of the parafollicular C cells is a precursor lesion of medullary thyroid cancer and a hallmark of multiple endocrine neoplasia type 2 (MEN2). (See "Medullary thyroid cancer: Clinical manifestations, diagnosis, and staging" and "Clinical manifestations and diagnosis of multiple endocrine neoplasia type 2".)

BLOOD SUPPLY — The arterial blood supply to the thyroid gland is primarily from the right and left superior and inferior thyroid arteries, derived from the external carotid arteries and thyrocervical trunk, respectively. The venous drainage consists of the superior, middle, and inferior thyroid veins that drain into the internal jugular vein and innominate vein (figure 5).

●Superior thyroid artery – The superior thyroid artery is the first branch off the external carotid artery. It extends inferiorly to the superior pole of the thyroid lobe. In addition to supplying the thyroid, the superior thyroid artery is the primary blood supply to approximately 15 percent of superior parathyroid glands. The superior thyroid artery is a landmark for identification of the superior laryngeal nerve, which courses with the artery until approximately 1 cm from the superior thyroid pole [14].

●Inferior thyroid artery – The inferior thyroid artery is a branch of the thyrocervical trunk that arises from the subclavian artery. The inferior thyroid artery courses posterior to the carotid artery to enter the lateral thyroid. The point of entry can extend from superior to inferior thyroid poles. The inferior thyroid artery also supplies the inferior parathyroid glands and approximately 85 percent of superior parathyroid glands.

The recurrent laryngeal nerve (RLN) may course anterior or posterior to the inferior thyroid artery. In some cases, the RLN may branch into both an anterior and posterior position.

●Thyroidea ima artery – A thyroidea ima artery is found in approximately 3 percent of individuals and arises from the aortic arch or innominate artery and courses to the inferior portion of the isthmus or inferior thyroid poles [1]. Surgical control of the thyroidea ima artery is essential during thyroidectomy. The thyroidea ima artery can be quite enlarged in patients with thyroid disease such as goiter or hyperthyroidism.

NERVE SUPPLY — It is critical that the superior and recurrent laryngeal nerves be routinely identified and protected in thyroid surgery to reduce the risk of injury (figure 6).

The surgeon may use a number of different techniques to aid in the identification of the nerves and potential variations. These include visual recognition, the use of anatomic landmarks, palpation, and intraoperative nerve monitoring (figure 7).

Visualization of the entire course of the nerves in the central compartment identifies multiple branches when they occur. It also reduces the chance of inadvertently ligating a nerve when it is more redundant or takes a more superior course prior to insertion into the trachea. The dissection along the nerves should be conducted in a manner to reduce the risk of thermal or traction injury.

Intraoperative nerve monitoring can be used as an adjunct to anatomic identification. The dissection should still continue until the nerve is visible because the nerve monitor may provide a false signal when it is in close proximity to, but not directly on, the nerve. (See "Thyroidectomy", section on 'Intraoperative nerve monitoring'.)

Superior laryngeal nerves — The right and left superior laryngeal nerves originate from the right and left vagus nerves as they exit the base of the skull [15]. The superior laryngeal nerve courses with the superior thyroid artery until approximately 1 cm before the artery enters the capsule of the superior pole of the thyroid.

Injury to the superior laryngeal nerve during surgery can be minimized by dissecting the superior thyroid vessels at the level of the thyroid capsule. Maintaining hemostasis is critical because the control of bleeding from the superior thyroid vessels with hemostats may put the superior laryngeal nerve at risk.

The superior laryngeal nerve consists of two primary branches:

External branch — The external branch of the superior laryngeal nerve is primarily motor in function [14]. It is the external branch that innervates the inferior constrictor and cricothyroid muscles.

The external branch travels with the superior thyroid artery until approximately 1 cm before the artery enters the superior thyroid pole. The external branch then divides into branches that enter the lateral inferior pharyngeal constrictor muscle and the cricothyroid muscle. A few smaller branches may be seen entering the superior thyroid.

The proximity of the nerve to the branches of the superior thyroid artery makes it vulnerable to damage during dissection; rates of injury are as high as 30 percent [16,17]. Care must be taken during the dissection of the superior thyroid vessels to prevent inadvertent injury to the superior laryngeal nerve by staying as close as possible to the thyroid capsule.

Internal branch — The internal branch of the superior laryngeal nerve is sensory to the larynx. The internal branch enters the larynx through the thyrohyoid membrane superior to the external branch.

Recurrent laryngeal nerve — The recurrent laryngeal nerve (RLN) provides both sensory and motor function to the larynx. This nerve is sensory to the subglottic region and innervates all muscles to the larynx except the cricothyroid muscle. The RLN provides motor function for both vocal cord abduction and adduction.

The RLN is associated with the inferior thyroid artery at approximately the junction of the lower and middle thirds of the thyroid gland. On the left, the RLN ascends in the tracheoesophageal groove and crosses deep to the inferior thyroid artery; on the right, the RLN crosses more obliquely and is oriented more laterally than caudally. While the nerve most often crosses deep to the inferior thyroid artery, documented variations include passing anterior to the artery as well as passing between branches of the inferior thyroid artery [18]. The RLN also may bifurcate or trifurcate prior to insertion in the cricothyroid muscle. The motor function for adduction and abduction is located in the anterior branches of the RLN [19].

Palpation of the tracheoesophageal groove and the junction with the inferior edge of the thyroid cartilage identifies the approximate location of RLN insertion into the trachea. The nerve itself may also be palpated in the tracheoesophageal groove.

Right recurrent laryngeal nerve — The right RLN originates from the right vagus nerve at the level of the subclavian artery. It courses posterior to the subclavian artery, taking a transverse course as it travels superiorly toward the lateral trachea and into the tracheoesophageal groove. It courses through the ligament of Berry and enters the larynx through the first tracheal ring, inferior to the cricothyroid muscle. It is common to see several branches of the right RLN as it approaches the trachea. The most anterior branch of the right RLN is the crucial motor branch, but all branches should be preserved [20].

Left recurrent laryngeal nerve — The left RLN originates from the left vagus nerve at the level of the aortic arch. It courses posterior to the aorta at the ligamentum arteriosum, taking a direct superior course toward the tracheoesophageal groove before entering the larynx in a similar fashion to the right laryngeal nerve. The left RLN thus courses more directly superiorly than the right nerve and enters the tracheoesophageal groove at a level lower than the right RLN, which takes a more transverse course.

Nonrecurrent laryngeal nerves — A nonrecurrent RLN may be present in nearly 1 percent of patients. When present, the nerve will course directly from the vagus nerve to the trachea.

Rarely, the right inferior laryngeal nerve may not recur and instead crosses transversely from the vagus nerve and behind the common carotid artery; in almost all instances, it enters the larynx at the usual level. Nonrecurrence of the inferior laryngeal nerve results from a vascular anomaly during embryonic development of the aortic arches in which the fourth vascular arch and right dorsal aorta involute, causing the aberrant development of arterial structures on the right, including the lack of an innominate artery and the passage of the subclavian artery behind the esophagus [21,22].

A nonrecurrent left inferior laryngeal nerve would require a right aortic arch associated with situs inversus and is exceedingly rare.

Nerve injury — Nerve injury can result from the disease process, surgery, or airway access for anesthesia. Injury to the superior laryngeal nerve results in voice weakness or fatigue as well as changes to both voice quality and pitch [23].

Injury to the RLN may result in paresis or paralysis of the true vocal cord to a paramedian or lateral position. Intrinsic muscles of the larynx, aside from the cricothyroid muscle, can be impaired, and the patient may have swallowing dysfunction with an aspiration risk. A bilateral injury to the RLN may necessitate a tracheostomy. These are discussed in detail elsewhere. (See "Thyroidectomy".)

LYMPHATIC DRAINAGE — The neck is divided into compartments based on patterns of lymphatic drainage (figure 8 and figure 9). The central compartment (level VI) is the primary lymphatic drainage region for the thyroid gland. The central compartment is bounded superiorly by the hyoid bone, laterally by the common carotid arteries, and inferiorly by the innominate artery just caudal to the suprasternal notch [24].

The lateral neck includes levels II through V. Level II is located between the angle of the jaw and base of the skull to the hyoid bone. Level III is from the hyoid bone to the cricoid cartilage. Level IV is from the omohyoid muscle to the clavicle. Level V is the posterior triangle that is bounded by the sternocleidomastoid muscle, trapezius muscle, and clavicle. Superior mediastinal nodes are the level VII nodes.

PARATHYROID GLANDS — The parathyroid glands are in close approximation with the thyroid gland and function independently of the thyroid gland. Identification of the superior and inferior parathyroid glands allows the surgeon to delineate the course of the recurrent laryngeal nerve (RLN), which lies anterior to the superior parathyroid gland and posterior to the inferior parathyroid gland.

The parathyroid glands are discussed in detail separately. (See "Surgical anatomy of the parathyroid glands".)

SUMMARY

●Thyroid gland size and location – The normal thyroid gland is immediately caudal to the larynx and encircles the anterolateral portion of the trachea and is bordered by the trachea and esophagus medially, the carotid sheath laterally, and the sternocleidomastoid, sternohyoid, and sternothyroid muscles anteriorly and laterally. (See 'Size and location' above.)

●Tubercle of Zuckerkandl – The tubercle of Zuckerkandl is located on the posterior aspect of each thyroid lobe. The recurrent laryngeal nerve (RLN) usually traverses the posterior aspect of the tubercle, which can help the surgeon find and identify the nerve. (See 'Tubercle of Zuckerkandl' above.)

●Blood supply – The arterial blood supply to the thyroid gland is primarily from the right and left superior and inferior thyroid arteries, derived from the external carotid arteries and thyrocervical trunk, respectively. The venous drainage consists of the superior, middle, and inferior thyroid veins that drain into the internal jugular vein and innominate vein. (See 'Blood supply' above.)

●Nerve innervation – The surgeon may use a number of different techniques to aid in the identification of the nerves. These can include direct visualization, the use of anatomic landmarks, palpation, and intraoperative nerve monitoring. (See 'Nerve supply' above.)

•Superior laryngeal nerve – The superior laryngeal nerve has an external and internal branch. The external branch is primarily motor in function and innervates the inferior constrictor and cricothyroid muscles. The internal branch is sensory in function to the larynx. (See 'Superior laryngeal nerves' above.)

•Recurrent laryngeal nerve – The RLN provides both sensory and motor function to the larynx. Injury to the RLN may result in paresis or paralysis of the true vocal cord to a paramedian or lateral position. Intrinsic muscles of the larynx, aside from the cricothyroid muscle, can be impaired, and the patient may have swallowing dysfunction with an aspiration risk. (See 'Nerve injury' above.)

●Lymphatic drainage – The primary lymphatic drainage region for the thyroid gland is the central compartment of the neck. (See 'Lymphatic drainage' above.)