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Thyroid

The thyroid gland is an endocrine gland secreting iodine-containing hormones and thyrocalcitonin.

thyroid
Thyroid gland: 1 - pyramidal lobe; 2 - left lobe; 3 - isthmus of the thyroid gland; 4 - inferior thyroid artery (top) and vein; 5 - the lowest thyroid artery (right - vein); 6 - right lobe; 7 — superior thyroid artery (below) and vein.

Anatomy . The thyroid gland is located on the front surface of the neck. It consists of two lobes and an isthmus (fig.). Sometimes from the isthmus the pyramidal share departs upwards. The thyroid gland has a fibrous capsule. With the help of ligaments, the capsule of the thyroid gland is fixed to the cricoid cartilage of the larynx and trachea , which causes displacement of the gland when swallowing, together with the larynx and trachea. The blood supply of the thyroid gland is the upper and lower thyroid arteries, the veins of the thyroid gland fall into the internal jugular and common facial veins.

In some cases, there may be an additional (lowest) thyroid artery and vein. Lymphatic vessels are sent to the deep cervical lymph nodes. The thyroid gland is innervated by branches of the vagus nerves and sympathetic trunks.

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The parenchyma of the thyroid gland consists of oval-shaped follicles, the cavity of which is filled with a colloid (protein-mucopolysaccharide mass, the main part of which is a specific iodine-containing protein - thyroglobulin). The wall of the follicle consists of a monolayer epithelium lying on the basement membrane.

Physiology . The thyroid gland secretes three hormones into the blood: two iodine-containing ones - thyroxine (tetraiodothyronine) and triiodothyronine, and the third, which does not contain iodine - thyrocalcitonin. Iodine enters the body with food and water; the thyroid gland concentrates from the blood iodides, whose iodine is used in the synthesis of hormones. Iodine-containing hormones are produced in the epithelial cells of the follicles, from where they enter the colloid; thyrocalcitonin is produced in interfollicular epithelial cells.

Thyroxine and triiodothyronine act unidirectionally, but the action of the first is slow, the second is faster and more active. Both of these hormones affect the consumption of oxygen by the tissues and are involved in the regulation of the metabolism of proteins, fats and carbohydrates , as well as water metabolism. The main biological processes depend on their activity: growth and development of the organism and tissue differentiation. Calcitonin regulates the exchange of calcium in the body and, preventing its resorption from bone tissue, promotes its utilization.

Thyroid activity is regulated by the nervous system and the pituitary gland. The hypothalamus produces a neurosecret (thyrotropin-realizing factor) that regulates the secretion of thyroid stimulating hormone of the pituitary gland; the latter regulates the activity of the thyroid gland.

The thyroid gland (glandula thyreoidea, s. Corpus thyreoideum) is an endocrine gland that synthesizes and accumulates iodine-containing hormones, which control the important physiological functions of the body (growth, development, metabolic rate, metabolism).

Embryology
The thyroid gland develops from the ventral wall of the pharyngeal intestine by the end of the 1st month. embryonic life in the form of a thickening of the ectoderm. From the apex of this thickening, the epithelial cord (the future thyroid-lingual duct) begins to grow, divided by the middle of the 2nd month of embryonic life into two lateral outgrowths. The latter, expanding, form lateral lobes, and the lower part of the thyroid gland is preserved in the form of an isthmus between them. Obliteration of the thyroid duct occurs in the 2nd month of fetal life, but sometimes a part of it can be saved and serve as a source for the development of additional thyroid glands. The epithelium of the lateral lobes of the thyroid gland grows in the form of cell strands, followed by differentiation into follicles. The first follicles appear in the embryo by the 4th month. embryonic life, signs of secretion are detected earlier. The development of the thyroid gland ends by the 8-9th month. fetal development.

Anatomy


Fig. 1. Pyramidal lobule and additional thyroid gland.
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The thyroid gland is located on the front surface of the neck (tsvetn. Table, Fig. 1), has a horseshoe-shaped or semi-lunar shape and consists of two lateral lobes and an isthmus. The isthmus is adjacent to the trachea in front, lateral lobes on the side (right and left). Sometimes there is a non-permanent pyramidal segment (Fig. 1). Occasionally the isthmus is absent and the lateral lobes are connected by a connective tissue web. The shape and size of the fractions are subject to significant variations (Fig. 2). In women, the thyroid gland is larger than in men. Its size increases with pregnancy. The upper boundary of the gland is the outer surface of the thyroid cartilage, the lower is the 5th – 7th ring of the trachea. The thyroid gland has external and internal connective tissue capsules (Fig. 3). The inner covers the gland, the outer forms a ligamentous apparatus, fixing the thyroid gland to the trachea and larynx. Between the sheets of capsules are vessels.

thyroid abnormalities
Fig. 2. Variants of the form and anomalies of the thyroid gland (according to Marshall).

where is the thyroid gland
Fig. 1. Topography of the thyroid gland (its relation to fascia, muscles and organs of the neck).

blood vessels of the thyroid gland
Fig. 2 and 3. Blood vessels of the thyroid gland (Fig. 2 — front view; Fig. 3 — rear view);
1 - v. brachiocephalica dext .;
2 - v. thyreoidea ima;
3 - plexus thyreoideus impar;
4 - a. subclavia;
5 - truncus thyreocervicalis;
6 - a. thyreoidea inf .;
7 - v. thyreoidea inf .;
8 - v. jugularis int .;
9 - v. thyreoidea sup .;
10 - a. thyreoidea sup .;
11 - a. carotis ext .;
12 - a. carotis int .;
13 - v. thyreoidea media;
14 - v. brachiocephalica sin .;
15 - plexus pharyngeus (venosus).

The thyroid gland is richly supplied with vessels: two superior (branches of the external carotid artery), two lower and one (intermittent) unpaired thyroid arteries (color. Table, fig. 2 and 3). The superior thyroid artery is divided into three branches: anterior, external, and posterior. The latter anastomoses with the lower thyroid artery. The inferior thyroid artery is divided into three terminal branches: lower, posterior, and deep. The venous system is more powerful than the arterial system, and is also rich in anastomoses.

The lymphatic system of the thyroid gland is represented by capillaries, superficial and deep lymphatic vessels. Lymph drainage is carried out in the jugular lymphatic trunk. The latter, at the level of the VII cervical vertebra, infuses into the left subclavian vein or its junction with the internal jugular vein.

The thyroid gland is innervated by branches of the vagus, sympathetic, laryngopharyngeal and hypoglossal nerves (Fig. 4), which are interconnected by anastomoses.

thyroid capsules
Fig. 3. Internal and external (passing into the fascial vagina of the neurovascular bundle) capsules of the thyroid gland.

thyroid nerves
Fig. 4. Thyroid nerves (half-circuit):
1 - n. vagus;
2 - n. laryngeus recurrens (with branches to the gland);
3 - n. laryngeus sup. (with branches to the gland);
4 - gangl. cervicale sup .;
5 - truncus sympathicus;
6 - sympathetic plexus on a. thyreoidea sup. (plexus tliyreoideus sup.);
7 - gangl. cervicale medium (with branches to the gland);
8 - sympathetic plexus on a. thyreoidea inf. (plexus thyreoideus inf.).

Histology
The parenchyma of the thyroid gland consists of follicles of a round or oval shape. The average value of 40-50 microns. The wall of the follicle is represented by a single-layer epithelium, located on the basement membrane, which is intimately connected with the endothelium of the capillaries. The height and shape of the epithelium vary depending on its functional activity. There are two types of follicular epithelium: cubic and prismatic. In the apical regions of epithelial cells there is a membrane that forms papillary outgrowths into the lumen of the follicle. The nuclei of the follicular epithelium are light, round or oval. The follicle cavity is filled with a colloid - a protein-mucopolysaccharide mass, the main part of which is a specific iodine-containing protein - thyroglobulin. In the cytoplasm of the follicular epithelium, the endoplasmic reticulum is well developed, which is a system of tubules, vacuoles, cisterns limited by cytoplasmic membranes. The shape of the network changes due to the functional activity of the cell. On the outer surface of the cytomembrane are ribosomes rich in protein and RNA. Ribosomes are in the form of rosettes or spirals. In the apical part and in the region of the nucleus of the follicular epithelium there are mitochondria. Their shape is varied - from round to rod-shaped. The size and number of mitochondria increase with active secretory activity of the thyroid gland. Above the core is the Golgi apparatus, which has a mesh structure and greatly increases during the period of secretory activity.

The cytoplasm of the follicular epithelium contains ascorbic acid, RNA, phosphatase (acidic, alkaline), iodine peroxidase, succinic dehydrogenase, cytochrome oxidase, and other enzymes. Among epithelial cells, parafollicular light cells and Langendorf cells with dark-colored cytoplasm are rarely found in the wall of the follicles. Both of them arise from the cells of the follicular epithelium and constitute one of the morphofunctional stages in a single cycle of its transformations. Parafollicular cells tend to increase and expand the Golgi apparatus network, insignificant phosphatase activity, low content of neutral mucopolysaccharides, histidine, and RNA.

The growth of the thyroid gland is due to the formation of follicles in the foci of the cambial interfollicular parenchyma and in the intrafollicular pads. The latter result from the proliferation of the epithelium into the lumen of the follicles. Secondary follicles are small, their cells retain the ability to produce colloid.