Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Endocrine System
Classification, structure, and functions of endocrine glands
Thyroid gland

The Thyroid Gland is located in the anterior region of the neck, in front of the Larynx and the upper cartilages of the Trachea. It consists of two lobes and an isthmus. In an adult, the gland weighs 20–30 g. The thyroid is externally covered by a Connective Tissue capsule that is fairly densely fused with the larynx, giving the gland mobility (along with the larynx). The parenchyma of the gland is composed of lobules, which in turn are formed by follicles. The walls of the follicles are lined with a single layer of thyrocytes, which are secretory endocrinocytes. Each follicle is enveloped by a dense network of Blood capillaries (Fig. 79). The follicular cavities contain a thick, viscous thyroid colloid, which is the secretory product of the thyrocytes. Thyrocytes secrete iodine-rich Hormones—thyroxine and triiodothyronine—collectively referred to as thyroglobulin. These hormones stimulate oxidative processes within the body's Cells. THYROID HORMONES influence protein, carbohydrate, fat, Water, and Mineral METABOLISM, as well as tissue growth, development, and differentiation.

Class="center">

Fig. 79. Structure of thyroid follicles (after Yu.I. Afanasyev and E.F. Kotovsky):

1 — blood capillaries; 2 — follicle; 3 — artery; 4 — glandular Cell; 5 — blood vessel; 6 — colloid containing thyroid hormones

An enhancement of thyroid function and an increased concentration of its hormones in the blood lead to a higher consumption of Proteins, fats, and CARBOHYDRATES. Consequently, a person consumes more food while rapidly losing weight.

The resulting increase in Energy Expenditure leads to rapid fatigue and exhaustion of the body. Sustained Hyperfunction of the thyroid gland (hyperthyroidism) causes a condition known as Graves' disease. The thyroid gland enlarges, a "goiter" (Swelling in the anterior neck region) appears, and symptoms such as tachycardia, irritability, sweating, and insomnia develop.

Hypofunction of the thyroid gland (hypothyroidism) in children leads to delayed physical and mental development, as well as impaired cognitive abilities. Puberty is delayed. In adults, hypothyroidism results in a severe condition known as Myxedema. Symptoms include rapid fatigue, drowsiness, dry Skin, and brittle Nails. The face becomes puffy due to subcutaneous edema, which may also spread to other PARTS OF THE body.

In regions where water and food are deficient in iodine—an essential component of thyroid hormones—a condition known as endemic goiter develops. This condition involves the proliferation of thyroid tissue, making the enlarged thyroid visible on the neck (a "goiter"). However, hormone production does not increase because the iodine deficiency impairs the synthesis of thyroxine and triiodothyronine. Such patients exhibit symptoms characteristic of hypothyroidism.

Large parafollicular cells are located within the follicular walls of the thyroid gland among the thyrocytes, as well as between the follicles. These cells synthesize and secrete the hormone thyrocalcitonin, which is involved in The regulation of Calcium and phosphorus metabolism; it also inhibits the release (resorption) of calcium from bones and decreases blood calcium levels.