Human Anatomy, Part 1 - K. A. Dyubenko, A. K. Kolomiytsev, Yu. B. Chaykovsky 2002

Special Section
Endocrine glands (ductless glands), glandulae endocrinae (glandulae sine ductibus)
Peripheral Endocrine Organs - Thyroid Gland

The Thyroid Gland, glandula thyroidea, is one of the large unpaired peripheral Endocrine glands. Its weight in an adult is 30-40 g (in a newborn, it is 1 g). It consists of the right, left, and pyramidal lobes, as well as the isthmus (Fig. 344).

Topography. The thyroid gland embraces the Larynx anteriorly and laterally, and superiorly it adjoins the thyroid Cartilage, from which it derived its name. The posterior surface of the gland is adjacent to the Pharynx, Trachea, and Esophagus, and partially covers the common carotid artery and the recurrent nerve. The anterior surface of the thyroid gland is covered by Skin, subcutaneous tissue, and the superficial Cervical fascia. Deeper down, it is covered by the superficial and deep layers of the deep cervical fascia.

The thyroid gland is enclosed in a fibrous capsule, capsula fibrosa, from which septa extend into the parenchyma, dividing it into lobules, lobuli. The Structural and functional unit of the thyroid gland is the follicle, which represents a spherical formation with a cavity filled with a protein colloid (thyroglobulin). The colloid contains 0.5% of iodine relative to its total amount. In the intestine, iodine compounds entering with Water and food are absorbed into the Blood as iodide, which reaches the gland and is utilized for thyroglobulin synthesis. Thyroglobulin accumulates within the follicular cavities and, depending on the body's metabolic needs, breaks down to yield two Hormones: triiodothyronine and tetraiodothyronine (thyroxine).

In Hyperfunction of the thyroid gland—hyperthyroidism, when excess thyroxine and triiodothyronine accumulate—the body's metabolic rate sharply increases. This leads to excessive heat production, tachycardia, cardiovascular manifestations (increased systolic blood pressure, regular episodes of tachycardia), and The Development of toxic goiter (Graves'-Basedow disease), which presents with characteristic signs: goiter, exophthalmos, weight loss, Muscle weakness, and tremor of the extremities.

In hypofunction of the thyroid gland—hypothyroidism, caused by a deficiency of THYROID HORMONES—cretinism develops in early childhood, manifesting as delayed growth, sexual and mental maturation, and Skeletal System disorders. In patients with hypothyroidism, a characteristic symptom is the development of so-called Myxedema (mucous edema), which imparts a distinct appearance to the face.

Along with the aforementioned hormones, a special type of Cells in the thyroid gland (calcitoninocytes) produces the hormone thyrocalcitonin (Calcitonin), which lowers blood calcium levels and acts as an antagonist to parathyroid hormone (the hormone of the Parathyroid glands).

Blood supply to the thyroid gland is provided by the right and left superior thyroid Arteries (Branches of the external carotid arteries) and the right and left inferior thyroid arteries (branches of the thyrocervical trunks). Occasionally, the thyroid gland receives blood from an unpaired lowest thyroid artery, a. thyroidea ima, which may originate from the brachiocephalic trunk, the Subclavian Artery, or the aortic arch.

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Fig. 344. Thyroid gland, glandula thyroidea (anterior view):

A: 1 - thyroid cartilage; 2 - pyramidal lobe; 3 - right and left lobes;

4 - ISTHMUS OF THE gland; 5 - trachea; Diagram of the thyroid gland Structure (according to E. F. Kotovsky): B: 1 - arteries; 2 - blood capillaries; 3 - follicle; 4 - glandular cells; 5 - colloid

Venous drainage from the thyroid gland is carried out by the superior and middle thyroid Veins, which empty into the Internal jugular vein of the corresponding side. The inferior unpaired thyroid vein empties into the brachiocephalic vein.

Lymphatic vessels drain Lymph from the thyroid gland into the deep cervical, pretracheal, paratracheal, and anterior mediastinal Lymph Nodes.

Innervation of the thyroid gland is provided by sympathetic nerve fibers from the cervical segment of the Sympathetic trunk. Parasympathetic fibers are supplied to the gland by the Vagus nerve via the laryngeal and recurrent nerves.