Anatomy - Total thyroidectomy for benign multinodular goiter on both sides.

Located between the anterior border of the sternocleidomastoid muscle, the mandible, and the jugular fossa, the anterior triangle of the neck contains the supra- and infrahyoid muscles, vessels, nerves, and the thyroid gland around the hyoid bone. For thyroidectomy, among the infrahyoid muscles, the medially located
• Sternohyoid muscle (sternum → hyoid bone) and beneath it the
• Sternothyroid muscle (sternum → thyroid cartilage of the larynx) as well as more laterally the
• Omohyoid muscle (scapula → intermediate tendon → hyoid bone)
are significant, as they partially cover the thyroid gland and must be retracted intraoperatively.

Blood Vessels
The common carotid artery divides at the upper edge of the thyroid cartilage at the level of its carotid sinus (receptors for blood pressure and chemoreceptors for blood gases) into its internal and external branches and runs immediately lateral in the carotid sheath next to the trachea and esophagus cranially. Here it touches the right and left thyroid lobes as a large vessel. The internal jugular vein arises from the sigmoid sinus of the skull, collecting blood from the skull and neck, initially attaching to the internal carotid artery in the carotid sheath, then running further laterally and enclosing the common carotid artery and the vagus nerve (X) laterally.

Nerves
In addition to the vagus nerve and its superior branch to the larynx (superior laryngeal nerve → anterior cricothyroid muscle and mucosa of the upper larynx), the deep cervical ansa (superior and inferior roots, from C1-C3) descends laterally in the thyroid region for the innervation of the three mentioned muscles of the anterior triangle of the neck and the transverse cervical nerve (from C2/3, skin innervation, platysma).

Fascial Layers
In the anterior triangle of the neck, there are several fascial layers under the skin (all belonging to the cervical fascia) that have certain peculiarities:
• The superficial lamina superficialis surrounds all neck structures except the platysma and separately encloses the sternocleidomastoid muscle and dorsally the trapezius muscle (accessory nerve XI),
• the middle lamina pretrachealis surrounds the infrahyoid muscles, and the
• deep lamina prevertebralis runs between the esophagus and spine outside the surgical area.

The thyroid/parathyroid glands and the trachea, as well as the lateral vascular-nerve bundle (carotid artery, internal jugular vein, and vagus nerve), also have their own organ fascias. The arrangement of the cervical fascias results in compartments enclosed by them and spaces (spatium) between them that extend into the mediastinum and represent potential infection pathways.

The unpaired thyroid gland (Glandula thyreoidea), along with the 4 parathyroid glands (Glandulae parathyroideae), belongs to the endocrine organs, located in the anterior neck region caudal and lateral to the thyroid cartilage. It is composed of a left and right lobe and an isthmus connecting both lobes. The lobes cover the lateral upper rings of the trachea as well as the cricoid and thyroid cartilage from the front, with the isthmus located centrally at the level of the 2nd/3rd tracheal ring. The organ has its own fibrous capsule (Capsula fibrosa) and is covered ventrolaterally by the infrahyoid muscles (M. sternohyoideus, M. sternothyroideus, and M. omohyoideus) as well as the pretracheal fascia. Due to its embryonic development and descent (via the thyroglossal duct) from the base of the tongue (Foramen caecum) to its later described location, cysts or ducts to the Foramen caecum, as well as functional glandular parts (e.g., as Lobus pyramidalis), can remain along this path.

Blood Supply
Arteries from the external carotid artery (→ A. thyreoidea superior) and from the subclavian artery via the thyrocervical trunk (→ A. thyreoidea inferior) supply the thyroid gland with anterior and lateral/posterior smaller branches. Inconsistently, but noteworthy, the A. thyreoidea ima approaches the gland directly from the aortic arch or the brachiocephalic trunk from below. The venous drainage, which has systemic significance for this endocrine organ, proceeds through similarly named vessels (V. thyreoidea superior and media → V. jugularis interna, V. thyreoidea inferior → V. brachiocephalica).

Lymphatic Vessels and Nerves
The lymphatic drainage from the thyroid gland occurs paratracheally and to deep cervical lymph nodes. Special attention should be paid intraoperatively to the recurrent laryngeal nerve (from the vagus nerve: running right around the subclavian artery, left around the aortic arch). It ascends in the groove between the trachea and esophagus to the larynx, where it innervates all laryngeal muscles except the cricothyroid muscle anteriorly (“Antikus”), in addition to providing sensory innervation to the mucosa of the lower half of the larynx.

The 4 parathyroid glands (Gld. parathyroideae) are small oval glands on the posterior surface of the thyroid lobes, supplied by the inferior thyroid artery.

During thyroidectomy, there is a risk of injuring structures such as the recurrent laryngeal nerve and the parathyroid glands located at the posterior aspect of the organ, as well as parts of the sympathetic trunk or vessels and nerves near the common carotid artery due to the close spatial proximity.

The paired, lentil-sized parathyroid glands are located between the fibrous capsule and the external capsule on the posterior surface of both thyroid lobes, occasionally embedded in the thyroid tissue. The upper parathyroid glands are usually at the level of the lower edge of the cricoid cartilage, the lower ones at the level of the 3rd to 4th tracheal cartilage.

The number and location of the parathyroid glands vary significantly; in 95% of cases, there are four or more. They can be found above and below the thyroid in the cervical connective tissue. Displacements cranially up to the level of the hyoid bone and caudally into the upper mediastinum occur.

The inferior thyroid artery supplies over 80% of the blood to the parathyroid glands. The rest is supplied by the superior thyroid artery and branches arising from the anastomoses between the two thyroid arteries.

The function of the parathyroid gland primarily involves the production and secretion of parathyroid hormone (PTH), a polypeptide consisting of 76 amino acids.

Parathyroid hormone regulates calcium metabolism by increasing renal reabsorption in the distal tubule. The release of PTH is mainly determined by the serum calcium level. The hydroxylation of 25-cholecalciferol to 1,25-dihydroxycholecalciferol, the active vitamin D3 metabolite, in the kidney is controlled by PTH and regulates the intestinal absorption of calcium and phosphate.

The majority of serum calcium is bound to albumin (80%), but only the free portion, the ionized calcium, is of physiological significance. The binding of calcium to albumin is pH-dependent and is enhanced by alkalosis.

PTH also directly influences serum levels of phosphate and magnesium through the regulation of bone calcium metabolism.