What is Thyroid: Anatomy, Blood Supply,Embyology,Muscle supply.

Anatomy Of Thyroid-

The thyroid gland is a midline structure " Butterfly Shaped" located in the anterior neck. The thyroid functions as an endocrine gland and is responsible for producing thyroid hormone and calcitonin, thus contributing to the regulation of metabolism, growth, and serum concentrations of electrolytes such as calcium.

Many disease processes can involve the thyroid gland, and alterations in the production of hormones can result in hypothyroidism or hyperthyroidism. The thyroid gland is involved in inflammatory processes (e.g., thyroiditis), autoimmune processes (e.g., Graves disease), and cancers (e.g., papillary thyroid carcinoma, medullary thyroid carcinoma, and follicular carcinoma).

In addition to considering its role in metabolism, growth, regulation of certain electrolytes, and its involvement in many disease processes, the thyroid gland deserves consideration for its anatomical location and its close relationship to important structures including the parathyroid glands, recurrent laryngeal nerves, and certain vasculature.

Anatomy Overview-

The thyroid gland is divided into two lobes that are linked by means of the isthmus, which crosses the midline of the upper trachea at the 2nd and 1/3 tracheal rings. In its anatomic position, the thyroid gland lies posterior to the sternothyroid and sternohyoid muscles, wrapping around the cricoid cartilage and tracheal rings. It is placed inferior to the laryngeal thyroid cartilage, normally corresponding to the vertebral stages C5-T1. The thyroid attaches to the trachea by way of a consolidation of connective tissue, referred to as the lateral suspensory ligament or Berry’s ligament. This ligament connects every of the thyroid lobes to the trachea. The thyroid gland, alongside with the esophagus, pharynx, and trachea, is observed inside the visceral compartment of the neck which is certain with the aid of pretracheal fascia.
The “normal” thyroid gland has lateral lobes that are symmetrical with a well-marked centrally placed isthmus. The thyroid gland commonly carries a pyramidal extension on the posterior-most factor of every lobe, referred to as the tubercle of Zuckerkandl. Despite these standard characteristics, the thyroid gland is acknowledged to have many morphologic variations. The function of the thyroid gland and its shut relationship with a variety of constructions brings about quite a few surgical concerns with medical relevance.

Embyology-

The parenchyma of the thyroid gland is derived from endoderm. The thyroid gland originates from the foramen cecum, which is a pit placed at the posterior one-third of the tongue. Early in gestation, the thyroid gland starts offevolved its descent anterior to the pharynx as a bilobed diverticulum. The thyroid gland then continues to descend the anterior of the hyoid bone and the cartilages of the larynx. By the seventh week, the thyroid gland reaches its vacation spot midline and anterior to the top trachea. The thyroglossal duct keeps the connection of the thyroid gland to the base of the tongue till involution and disappearance of the duct.
The ultimobranchial body, derived from the ventral place of the fourth pharyngeal pouch, then will become integrated into the dorsal factor of the thyroid gland. The ultimobranchial physique offers upward shove to the parafollicular cells or C cells of the thyroid gland.

Blood Supply and Lymphatics-

 The thyroid gland has an extremely rich blood supply and is estimated to be six times as vascular as the kidney and relatively three to four times more vascular than the brain.  

1.Superior and inferior thyroid arteries supplies the superior and inferior aspect of the gland.

2.The superior thyroid artery is the first branch of the external carotid artery as it arises near the level of the superior horn of the thyroid cartilage.

3.The superior thyroid artery then moves anterior, inferior, and towards the midline behind the sternothyroid muscle to the superior pole of the lobe of the thyroid gland. From this point, the superior thyroid artery branches off. One branching point runs down the dorsal aspect of the thyroid gland. The other superficial branch runs along the sternothyroid muscle and thyrohyoid muscles, supplying branches to these muscles as well as the sternohyoid. 
4.The superficial branch continues downward to further give off the cricothyroid branch and to supply the isthmus, inner sides of the lateral lobes, and when present, the pyramidal lobe.

5.The thyrocervical trunk arises from the anterosuperior surface of the subclavian artery and gives rise to three branches, one being the inferior thyroid artery.
6. The inferior thyroid artery branches from the thyrocervical trunk at the inner border of the anterior scalene muscle and advances medially to the thyroid gland. 
7.The artery reaches the posterior surface of the lateral lobe of the thyroid gland at the level of the junction of the upper two thirds and lower third of the outer border. 
8.The largest branch of the inferior thyroid artery is the ascending cervical branch, and it is important not to mistake this branch for the inferior thyroid artery itself.

9. In 10% of the population, there is an additional artery known as the thyroid ima artery. This artery has a variable origin including the brachiocephalic trunk, aortic arch, the right common carotid, the subclavian, the pericardiacophrenic artery, the thyrocervical trunk, transverse scapular, or internal thoracic artery. The thyroid ima most commonly originates from the brachiocephalic trunk and supplies the isthmus and anterior thyroid gland.

10.The thyroid gland is drained via the superior, middle, and inferior thyroid veins. The middle and superior thyroid veins follow a tortuous route and eventually drain into the internal jugular vein on either side of the neck. The drainage of the inferior thyroid vein may enter either the subclavian or brachiocephalic veins, located just posterior to the manubrium.

Lymphatic drainage of the thyroid gland involves the lower deep cervical, prelaryngeal, pretracheal, and paratracheal nodes. The paratracheal and lower deep cervical nodes, specifically, receive lymphatic drainage from the isthmus and the inferior lateral lobes. The superior portions of the thyroid gland drain into the superior pretracheal and cervical nodes. 

Musle Supply-

• Platysma: The first muscle encountered during neck dissection, it is enveloped by the superficial cervical fascia. It sits in the anterior neck and extends from the superficial fascia of the deltoid, over the clavicle, reaching the mandible and superficial fascia of the face superiorly.

• Sternocleidomastoid: This muscle forms the anterior portion of the posterior triangle of the neck. The muscle runs obliquely from the mastoid to the clavicle and sternum. The sternocleidomastoid is found anterolaterally relative to the thyroid gland.

• Digastric muscle: This muscle extends from the mandibular tubercle, passes deep and inferior to the hyoid, and loops back up to attach to the mastoid tip.

• Infrahyoid muscles: These are also referred to as “strap muscles.” They include four paired muscles found on the anterolateral surface of the thyroid gland. The strap muscles result in gross movement of the larynx during swallowing and also adjust the positioning of the larynx during vocalization.

• Omohyoid muscle: The omohyoid muscle is found deep in the sternocleidomastoid. It extends from the hyoid bone to the lateral aspect of the clavicle.

• Sternohyoid muscle: This muscle sits anterior the remaining strap muscles and the thyroid gland. The sternohyoid muscle extends from its superior attachment at the hyoid bone inferiorly to the sternum.

• Sternothyroid muscle: This muscle extends from the oblique line of the thyroid cartilage to the sternum. This muscle contacts the anterior surface of the thyroid gland.

• Thyrohyoid muscle: The thyrohyoid muscle extends from the oblique line of the thyroid cartilage to the hyoid bone superiorly.

•  Inferior pharyngeal constrictor: This muscle extends from its anterior attachment at the oblique line of the thyroid cartilage and lateral aspect of the cricoid cartilage to the pharyngeal raphe. This muscle contacts the superior pole of the lateral lobe of the thyroid gland medially. 

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