The follicles are embedded within the meshwork of reticular fibers Figure 1a [ 9 ]. The thyroid follicles are the main functional and structural components of the gland which synthesize and release T3 and T4 in the center of follicles. Each follicle is filled with colloid, which is a gelatinous substance containing the stored form of T3 and T4. In active glands, the colloid is predominantly basophilic, whereas in inactive glands, it is acidophilic.
In highly activated glands, this colloid is not only reduced in amount but also shows vacuoles [ 9 ]. There are types of thyroid cells, i. The follicular or principal cells are responsible for T3 and T4 production. These cells are usually simple cuboidal cells but may change to simple squamous inactive or columnar cells active depending on their states of secretion Figure 1b. Golgi apparatus is located in the supranuclear position. Ultrastructurally, the cells contain the organelles showing both secretory and absorptive characteristics and short microvilli on the apical surface of cells.
In basal location, cells contain a large number of rough endoplasmic reticulum. In apical location, cells contain small vesicles morphologically related to Golgi apparatus and a large number of endocytotic vesicles lysosomes defined as colloidal resorption droplets [ 10 ].
Parafollicular or clear cells C cells are the second type of thyroid cells, located within the follicular epithelium or as small clumps adjacent to the follicles. These cells are relatively large oval or ellipsoid cells with round nuclei and pale cytoplasm and are found lying on the basal follicular membrane. These cells produce calcitonin hormone released in response to high blood calcium and inhibits the activity of the osteoclasts [ 11 ]. The thyroid gland is enveloped by the fascia consisting of the anterior and posterior parts of the deep cervical fascia.
The gland weighs approximately 10—20 g, and each lobe measures an average of 5 cm in length, 2. The gland is slightly heavier and bigger in size during menstruation and pregnancy [ 13 ]. Thyroid lobes are located lateral to the trachea and esophagus, anteromedial to the carotid sheath, and posteromedial to the strap muscles sternohyoid, sternothyroid, and superior belly of the omohyoid and are innervated by the ansa cervicalis ansa hypoglossi , overlying from the level of the fifth cervical vertebra down to the first thoracic vertebra Figure 2a [ 13 , 14 ].
The shape of the gland varies from an H to a U form, consisted of two elongated lateral lobes with superior and inferior poles that are joined at the midline by an isthmus. The length of the isthmus is in between 12 and 15 high, connecting the two lobes. Occasionally, the isthmus may be absent, and the gland exists as two separate lobes Figure 2b.
The most lateral extension of the thyroid lobes is the Zuckerkandl tubercles ZTs. These tubercles are condensed thyroid parenchyma located in the cricothyroid junction, at the junction point of the medial thyroid with the ultimobranchial bodies, and have an important vicinity with the recurrent laryngeal nerve RLN.
ZTs develop from the embryologic fusion of the ultimobranchial body with the median anlage and the lateral thyroid anlages of the fourth pharyngeal pouch. The dissection of this tissue is important because the RLN is located below the ZTs located in the posterolateral of the thyroid gland [ 15 , 16 ].
Thyroglossal duct extends along the path of thyroid descending from the foramen cecum at the base of the tongue to the lower neck. The cysts of this duct are the most commonly encountered congenital cervical anomalies in humans.
They are usually asymptomatic but occasionally become infected by oral bacteria. A thin layer of the front and back of the deep cervical fascia wraps the thyroid lobes. This fascia joins the capsule by two suspensory ligaments, namely, the anterior and posterior suspensory ligaments. The anterior suspensory ligament extends from the superior medial aspect of each thyroid lobe to the cricoid and thyroid cartilage.
The posterior ligament, known as the Berry ligament, connects the thyroid to the cricoid cartilage and upper rings of the trachea. The ligament of Berry is closely attached to the cricoid cartilage and has important surgical implications due to its connection to the RLN.
The RLN usually enters deep into the posterior suspensory ligament [ 14 ]. During the retracting of the thyroid gland on the medial side, it should not be compelling, because it may cause RLN to be stretched and injured. In addition, rupture of the vena thyroidea media may occur bleeding. Care should be taken not to cause nerve damage during the dissection and hemostasis to control bleeding.
There are two superior and two inferior parathyroid glands. The parathyroid glands are small structures adjacent to or occasionally embedded in the thyroid gland. Usually, two pairs of parathyroid glands lie in proximity to the thyroid gland. The inferior glands migrate further and have more chance of being in ectopic sites [ 19 , 20 ]. The thyroid gland is a highly vascular organ, among other endocrine organs, in a sense that there is a rich blood flow with large amounts of anastomosis in the gland.
Arterial supply is bilateral from both the external carotid system and superior thyroid artery and subclavian system with the lower thyroid branch of the thyrocervical trunk. It may be a single thyroid ima artery arising from the brachiocephalic artery [ 21 ]. The superior thyroid arteries originate from the ipsilateral external carotid arteries and are divided into anterior and posterior branches in the apex of the thyroid lobes.
Inferior thyroid arteries originate from the thyrocervical shortly after the origin of the subclavian arteries. The inferior thyroid arteries extend from the neck to the back of the carotid sheath and enter the thyroid lobes at the midpoints. The inferior thyroid artery passes through the recurrent laryngeal nerve RLN and requires the identification of RLN before the arterial branches are ligated.
The inferior thyroid artery provides an arterial supply of the cervical esophagus with subclavian artery and branches directly from the aorta, intercostal arteries, and tracheobronchial arteries [ 22 ]. There are three main venous pathways of the thyroid: superior, middle, and inferior thyroid veins.
The superior thyroid vein accompanies the superior thyroid artery and drains to the internal jugular vein but not accompanied by the middle thyroid vein.
There are several inferior thyroid vessels that frequently flow into the internal jugular or brachycephalic veins [ 12 ]. RLN is a branch of the vagus nerve, responsible from the laryngeal motor function and feeling. The left RLN is looped from the vagus nerve to the back of the aorta, and the right RLN revolves around the right subclavian artery. During thyroidectomy, since these nerves rise along the trachea near the thyroid gland, the surgeon should pay attention to protect them.
The inferior thyroid artery and its terminal branches are closely related to the RLN at the entrance point of the thyroid gland. Sometimes, the nerve can be confused with a branch of the artery. Compared to the artery, it is less regular, rounded, and elastic [ 23 ]. A small, red, curved vein called vasa nervorum is usually seen in the wall of the nerve. The left RLN rises straight along the tracheoesophageal groove, while the right RLN is more inclined and lateral than the left one.
However, numerous variations have been defined, so care should be taken in every case. In the two upper tracheal rings, the RLN is embedded at the back of the suspensory ligament, called the Berry ligament. This ligament extends to backward of the recurrent nerve and tightly connects the thyroid to the trachea and esophagus. At this point, there is a posterior artery near the recurrent nerve, which gives a small branch to the thyroid gland and is not easy to be attached to this artery Figure 3 [ 23 , 24 ].
Thyrid gland location with nerve and parathyroid gland. The superior laryngeal nerve is also a branch of the vagus nerve. On the pharynx side, the internal carotid descends from the back of the artery and is divided into two arms: the external laryngeal nerve as the motor nerve and the internal laryngeal nerve as the sensory nerve. The superior laryngeal nerve contributes to the pitch of voice, and its paralysis can lead to significant contraction of pitch range, vocal fold vibratory phase asymmetry, and acoustic aperiodicity, thus leading to an overall poor vocal quality [ 24 ].
There is a close relationship between the superior thyroid artery and the external branch of the superior laryngeal nerve. This nerve injury may cause high-pitched noises. In order to prevent damage to the external branch of the superior laryngeal nerve, it is recommended to ligate the superior thyroid arteries as low as possible during thyroidectomy.
The cricothyroid artery, a branch of the superior thyroid artery, is located in the cephalic portion of the upper pole and moves toward the midline on the cricothyroid ligament. This vessel may be damaged during cricothyroidotomy and may cause bleeding.
Care should be taken in large area hemostasis to control bleeding. Ligating the veins one by one prevents nerve damage. Classification of the external branch of the superior laryngeal nerve according to the risk of potential damage [ 12 ] is given below. Type 1: The nerve crosses the superior thyroid vessels more than 1 cm above the border of the thyroid upper pole.
Type 2a: The nerve crosses the vessels less than 1 cm above the border of the thyroid upper pole. The recurrent laryngeal nerve on the right, after exiting the superior thoracic cavity may be located in the neck root, in the lateral carotid artery, in the medial trachea, and in the triangle formed by the superior thyroid lobe.
The lower parathyroid glands are located proximal to the inferior laryngeal nerve, and the upper parathyroid glands are located distal to the nerve. Typically, extralaryngeal branching is in two forms as motor and sensory branch. However, two to eight extralaryngeal branches have been described in the literature. Its linear extension and its light yellow color make it known macroscopically. The right inferior laryngeal nerve is 32 cm long, and the left is approximately 43 cm long.
Since the left inferior laryngeal nerve has a longer course in the tracheoesophageal groove, the majority of nerve injuries occur in this side. Nonrecurrent laryngeal nerve was reported in 0. Nonrecurrent laryngeal nerve exits the cervical section of the vagus at the level of the larynx or thyroid gland and directly enters the larynx at the level of the cricothyroid joint without forming a loop [ 25 ].
The intraoperative methylene blue spraying technique could be used in thyroid surgery. Methylene blue will be sprayed over the thyroid lobe and perilober area. The connection of the diverticulum with the pharynx is termed the thyroglossal duct; its continuity is subsequently interrupted, and it undergoes degeneration, its upper end being represented by the foramen cecum of the tongue, and its lower by the pyramidal lobe of the thyroid gland.
When the organ is cut into, it is of a brownish-red color, and is seen to be made up of a number of closed vesicles, containing a yellow glairy fluid, and separated from each other by intermediate connective tissue.
The vesicles of the thyroid of the adult animal are generally closed spherical sacs; but in some young animals, e. This appearance is supposed to be due to the mode of growth of the gland, and merely indicates that an increase in the number of vesicles is taking place. Each vesicle is lined by a single layer of cubical epithelium.
There does not appear to be a basement membrane, so that the epithelial cells are in direct contact with the connective-tissue reticulum which supports the acini. The vesicles are of various sizes and shapes, and contain as a normal product a viscid, homogeneous, semifluid, slightly yellowish, colloid material; red corpuscles are found in it in various stages of disintegration and decolorization, the yellow tinge being probably due to the hemoglobin, which is thus set free from the colored corpuscles.
The colloid material contains an iodine compound, iodothyrin, and is readily stained by eosin. IMAIOS and selected third parties, use cookies or similar technologies, in particular for audience measurement.
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The website cannot function properly without these cookies, which is why they are not subject to your consent. These are cookies intended to measure the audience: it allows to generate usage statistics useful for the improvement of the website. Verify now. Toggle navigation. Institutional subscriptions support Language. Keep me signed in. Forgot your password? Sign in with Facebook. Sign in with Apple. Description The thyroid gland is a highly vascular organ, situated at the front and sides of the neck; it consists of right and left lobes connected across the middle line by a narrow portion, the isthmus.
The deep or medial surface is moulded over the underlying structures, viz. The anterior border is thin, and inclines obliquely from above downward toward the middle line of the neck, while the posterior border is thick and overlaps the common carotid artery, and, as a rule, the parathyroids. Vessels and Nerves: The arteries supplying the thyroid gland are the superior and inferior thyroids and sometimes an additional branch thyroidea ima from the innominate artery or the arch of the aorta, which ascends upon the front of the trachea.
The arteries are remarkable for their large size and frequent anastomoses. The veins form a plexus on the surface of the gland and on the front of the trachea; from this plexus the superior, middle, and inferior thyroid veins arise; the superior and middle end in the internal jugular, the inferior in the innominate vein. The capillary bloodvessels form a dense plexus in the connective tissue around the vesicles, between the epithelium of the vesicles and the endothelium of the lymphatics, which surround a greater or smaller part of the circumference of the vesicle.
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