Physiology of the Thyroid Gland
The thyroid gland (glandula thyreoidea) consists of two lobes and the isthmus connecting them. The isthmus can be absent and then the lobes are connected by a thin connective tissue jumper. In 30% of people there is a pyramidal process, which is a narrow part of the gland, a pyramidal or cone-shaped form, of various sizes. The pyramidal process usually departs not exactly from the midline, but from the angle between the isthmus and the lobe, more often the left one, and extends upwards in front of the thyroid cartilage, sometimes up to the hyoid bone.
The thyroid gland is covered with two fascial leaves. The inner leaflet or thy thyroid capsule is a thin fibrous plate that fuses with the parenchyma of the gland and, by sending the appendages into the thickness of the gland, divides it into separate lobules does synthroid cause weight loss. The so-called outer sheet is a visceral leaf IV of the fascia of the neck, which forms fascial.
Vagina for the innards of the neck - pharynx, esophagus, larynx, trachea and thyroid gland. Between the capsule of the thyroid gland and the vagina there is a slit-like space filled with loose fiber, which contains arteries, veins, nerves and parathyroid glands. The parietal leaflet of the IV fascia lies in front and from the sides of the innards of the neck and forms the vagina of the neurovascular bundle of the neck on both sides.
Thus, the thyroid capsule does not have an external leaflet. This circumstance has great practical significance. The fact is that the debate about the method of treating the vessels of the thyroid gland (intracapsular or extra-capsular) is explained only by a misconception about the anatomical features of fascial leaflets. Dense fibers of the IV fascia form ligaments that fix the thyroid gland to the larynx and trachea.
The thyroid gland covers the trachea in a horseshoe manner. The isthmus lies in front of the trachea (at the level from the first to the third or from the second to the fourth of its cartilages, and often covers part of the cricoid cartilage). Its lower edge, in the normal position of the head, is 1.5-2 cm from the jugular notch in the adult. The lower poles of the lobes reach the level of the 5th or 6th tracheal rings, and the upper poles reach the boundary between the middle and lower third Thyroid cartilage. The cervical region of the esophagus lies first along the middle line, behind the trachea, and at the C7 level (this is the level of the lower edge of the cricoid cartilage) deviates to the left.
Therefore, the left share of the thyroid gland is often attached to the esophagus wall (especially if the proportion is increased). The lateral lobes of the gland partially cover the common carotid artery, which often forms a depression in the form of a groove on the posterior surface of the lobe. The enlarged lobe of the gland pushes the inner jugular vein outward, which can be flattened on the surface of the lobe. From the front, the thyroid gland is covered by the sternum-hyoid, the scapula-hyoid and the sternum-thyroid muscles (mm.sternohyoidei, sternothyreoidei and omohyoidei).
Blood supply to the thyroid gland is carried out by four arteries - paired upper and lower thyroid arteries. Sometimes there is still an unpaired thyroid artery. The upper thyroid artery (a thyreoidea superior) usually departs from the trunk of the external carotid artery near the site of its division, less often from the bifurcation of the common carotid artery. Sometimes it forms a common trunk with a sublingual artery. Approaching the upper pole of the thyroid gland, the artery divides into the outer, inner, posterior and anterior branches.
Usually there are 2-3 branches, usually three. The most constant and largest anterior branch, which is clearly visible at the exposed upper pole of the lobe of the gland. It is easy to take for the main trunk. The internal branch passes along the upper edge of the isthmus and anastomizes with the same branch of the opposite side. Surgical anatomy and physiology of the thyroid gland. The lower thyroid artery (a. Thyreoidea inferior) departs from thyroid-cervical trunk (truncus thyreo-cervicalis) and is its largest branch.
In rare cases, it departs directly from the subclavian artery. The lower thyroid artery goes first up the front surface of the anterior staircase to the level of the cricoid cartilage, then, forming an arc, turns inward and obliquely downward, reaching the lower third of the posterior surface or the lower pole of the lobe of the gland.
Here the artery breaks down into three branches: the lower one, which runs along the lower edge of the lateral lobe and the isthmus to the middle line, where it anastomizes with the same branch of the other side; Upper, which rises along the posterior margin of the lateral lobe and anastomoses with the branch of the upper thyroid artery, and a deep gland that extends into the parenchyma. During its horizontal course, the lower thyroid artery crosses several important anatomical formations: the common carotid artery, which lies in front, the vertebral artery and the veins that lie behind it, the trunk of the sympathetic nerve that passes in front of and behind the artery and lower laryngeal (return) nerve.
The lower thyroid artery may be absent (2-3%). Unpaired artery (a. thyreoidea ima) is observed in approximately 10% of cases, often with a poorly developed lower thyroid artery. Usually it starts from an unnamed artery (a. Anonyma), sometimes from the right common carotid artery or from the arch of the aorta. There are anastomoses between the same upper and lower arteries, as well as longitudinal anastomoses that connect the upper and lower arteries, usually a posterior longitudinal anastomosis. The most constant anastomosis is located along the upper edge of the isthmus arterial arch, connecting among themselves all four arteries.
Thyroid arteries form two systems of collaterals intraorganic (due to thyroid arteries) and extraorganic (due to anastomoses with vessels of the pharynx, esophagus, larynx, trachea and adjacent muscles). Thyroid veins are more numerous than arteries. The upper thyroid vein accompanies the same-named artery and often, together with the hyoid vein, flows into the common facial vein. The lower thyroid veins are very variable.
The veins of the gland form a broad venous plexus, the most developed in the region of the lower edge of the isthmus and the anterior surface of the trachea is the unpaired venous plexus (plexus venosus thyreoideus impar). The upper and middle thyroid veins flow into the internal jugular vein, and the lower ones into the brachiocephalic vein.
The innervation of the thyroid gland is carried out by the branches of the sympathetic, wandering, and sublingual and lingopharyngeal nerves. The relationship of the thyroid gland with the laryngeal nerves. Hortus nerves are branches of the vagus nerve and have a mixed structure containing motor and sensitive fibers. The recurrent guttural nerves (n. Laryngei recurrentes) move away from the vagus nerves and, circling the arch of the aorta to the left. On the right, the nerve passes more superficially and outwards.
Both the nerve of the lower horns of the thyroid cartilage enters the larynx. In 60% of cases the nerve enters the larynx with one trunk, and in 40% of the observations at the level of the lower pole of the gland is divided into two or more branches. Of great practical importance is the level of division of the recurrent nerve and its relationship with the lower thyroid artery. The superior laryngeal nerve (laryngeus superior) has two branches, external and internal.
Outer branch (motor) innervates the laryngeal mucosa and the pusthneshchitovidnuyu muscle, which takes part in the movement of the vocal cords. The inner branch (sensitive) innervates the larynx mucosa, epiglottis and partially the root of the tongue.
When tying the upper thyroid artery, the upper laryngeal nerve can be damaged, especially if the upper pole of the lobe is high. Damage to the external branch of the nerve leads to a restriction of the movements of the corresponding vocal cords, and trauma of the inner branch to the paresis of the epiglottis.