Thyroid

The thyroid gland (glandula thyroidea) is an unpaired organ located in the anterior region of the neck at the level of the larynx and the upper part of the trachea. The gland consists of two parts - the right (lobus dexter) and left (lobus sinister), connected by a narrow isthmus. The thyroid gland is rather superficial. In front of the gland, below the hyoid bone, are paired muscles: the sternum-thyroid, the sterno-hyoid, the scapular-hyoid, and only partly the sternocleidomastoid muscle, and the superficial and pre-tracheal plate of the cervical fascia.

The posterior concave surface of the gland extends from the front and sides of the lower larynx and the upper part of the trachea. The isthmus of the thyroid gland (isthmus glandulae thyroidei), connecting the right and left lobes, is, as a rule, at the level of the II or III cartilage of the trachea. In rare cases, the isthmus of the gland lies at the level of the cartilage of the trachea or even the arch of the cricoid cartilage. Sometimes the isthmus can be absent, and then the lobes of the gland are not connected at all with each other.

The upper poles of the right and left lobes of the thyroid gland are located somewhat lower than the upper edge of the corresponding plate of the thyroid cartilage of the larynx. The lower pole of the lobe reaches the level of the V-VI cartilage of the trachea. The zadnobokovaya surface of each lobe of the thyroid gland comes into contact with the throat part of the pharynx, the beginning of the esophagus and the anterior semicircle of the common carotid artery. Parathyroid glands adhere to the posterior surface of the right and left lobes of the thyroid gland.

From the isthmus or from one of the lobes extends upwards and lies in front of the thyroid cartilage the pyramidal part (lobus pyramidalis), which occurs in about 30% of cases. This part sometimes reaches the body of the hyoid bone by its apex.

The transverse size of the thyroid gland in an adult reaches 50-60 mm. The longitudinal dimension of each lobe is 50-80 mm. The vertical size of the isthmus varies from 5 to 2.5 mm, and its thickness is 2-6 mm. The weight of the thyroid gland in adults from 20 to 60 years is on average 16.3-18.5 g. After 50-55 years, there is a slight decrease in the volume and mass of the gland. The mass and volume of the thyroid gland in women is greater than that of men.

Outside, the thyroid gland is covered with a connective tissue membrane - a fibrous capsule (capsula fibrosa), which is fused to the larynx and trachea. In connection with this, the movement of the thyroid gland occurs during movements of the larynx. Inside the gland from the capsule, connective tissue septa - trabeculae separating the gland tissue into lobules, which consist of follicles. Walls of follicles from within are lined with epithelial follicular cells of cubic form (thyrocytes), and inside the follicles is a dense substance - a colloid. The colloid contains hormones of the thyroid gland, consisting mainly of proteins and iodine-containing amino acids.

The walls of each follicle (about 30 million of them) are formed by a single layer of tyrots located on the basal membrane. The size of the follicles is 50-500 microns. The form of thyrocytes depends on the activity of synthetic processes in them. The more active the functional state of the thyrocyte, the higher the cell. Thyrots have a large nucleus in the center, a significant number of ribosomes, a well-developed Golgi complex, lysosomes, mitochondria and secretion granules in the apical part. The apical surface of the thyrocytes contains microvilli immersed in a colloid located in the cavity of the follicle.

The glandular follicular epithelium of the thyroid gland is greater than other tissues and has a selective ability to accumulate iodine. In the tissues of the thyroid gland, the concentration of iodine is 300 times higher than its content in the blood plasma. Thyroid hormones (thyroxine, triiodothyronine), which are complex compounds of iodinated amino acids with protein, can accumulate in the colloid of follicles and, if necessary, be released into the bloodstream and delivered to organs and tissues.

Thyroid hormones

Thyroid hormones regulate metabolism, increase heat exchange, enhance oxidative processes and consumption of proteins, fats and carbohydrates, promote the release of water and potassium from the body, regulate growth and development processes, activate the activity of the adrenal, sex and breast glands, have a stimulating effect on the activity of the central nervous system.

Between the thyrocytes on the basement membrane, as well as between the follicles, there are parafollicular cells, the apexes of which reach the lumen of the follicle. Parafollicular cells have a large rounded nucleus, a large number of myofilament in the cytoplasm, mitochondria, the Golgi complex, a granular endoplasmic reticulum. In these cells, there are many granules of high electron density about 0.15 μm in diameter. Parafollicular cells synthesize thyrecalcitonin, which is a parathyroid hormone antagonist, a parathyroid hormone. Thyreocalcitonin is involved in the exchange of calcium and phosphorus, reduces the calcium content in the blood and delays the release of calcium from the bones.

Regulation of thyroid function is provided by the nervous system and thyrotropic hormone of the anterior lobe of the pituitary gland.

Thyroid embryogenesis

The thyroid gland develops from the epithelium of the anterior colon in the form of an unpaired median outgrowth at the level between the I and II visceral arches. Until the 4th week of embryonic development, this outgrowth has a cavity, which is why it was called the thyroid duct (ductus thyroglossalis). By the end of the fourth week this duct atrophies, and its beginning remains only in the form of a more or less deep blind hole at the border of the root and body of the tongue. The distal duct is divided into two rudiments of future lobes of the gland. The forming parts of the thyroid gland are displaced caudally and occupy their usual position. The preserved distal part of the thyroid duct becomes a pyramidal part of the organ. Reducing sections of the duct can serve as a rudiment for the formation of additional thyroid glands.

Vessels and nerves of the thyroid gland

Respectively, the right and left upper thyroid arteries (branches of the external carotid arteries), and the right and left lower thyroid arteries (from the thyroid shafts of the subclavian arteries) to the lower poles of these lobes correspond respectively to the upper poles of the right and left lobes of the thyroid gland. Branches of the thyroid arteries form in the capsule of the gland and numerous anastomoses inside the organ. Sometimes the lower terminal of the thyroid gland fits the so-called lower thyroid artery, which extends from the brachiocephalic trunk. Venous blood from the thyroid gland drains over the upper and middle thyroid veins into the internal jugular vein, along the lower thyroid vein into the brachiocephalic vein (or into the lower section of the internal jugular vein).

Lymphatic vessels of the thyroid gland enter the thyroid, pre-horten, pre- and paratracheal lymph nodes. The nerves of the thyroid gland go away from the cervical nodes of the right and left sympathetic trunks (mainly from the middle cervical node, go along the vessels), and also from the vagus nerves.

Age features of the thyroid gland

The size of the thyroid gland in a newborn is much larger than that of a fetus. During the first year of life there is a slight decrease in the mass of the thyroid gland, which reaches 1.0-2.5 g. Until puberty, the size and mass of the thyroid gland gradually increase (to 10-14 g). In the period from 20 to 60 years the mass of the organ does not change significantly, remains almost constant and is equal to an average of 18 g. Some decrease in the mass and size of the organ due to age atrophy occurs in old age, but the function of the thyroid gland in old age often remains intact.

[1], [2], [3], [4], [5], [6]