The thyroid gland is located in the neck anterior to the larynx and consists of two lobes connected by an isthmus. In the early embryonic period, it develops from the endoderm of the initial part of the primary intestine. Its function is to synthesize the hormones thyroxine (T) and triiodothyronine (T3), which are necessary for cell growth and differentiation, regulation of oxygen consumption and basic metabolism in the body. Thyroid hormones affect the metabolism of proteins, lipids and carbohydrates. Thyroid tissue consists of 20-30 million microscopic spherical structures known as thyroid follicles, or thyroid follicles. The follicles are lined with a single-layer epithelium, and the cavity inside contains a jelly-like substance – a colloid. The thyroid gland is the only endocrine gland whose secretory product is stored in very large quantities. This process is also unusual in that hormones accumulate extracellularly in the colloid. In humans, the hormone content in the follicles is sufficient to meet the needs of the body for a period of up to 3 months. Thyroid colloid (thyroid colloid) includes a high molecular weight glycoprotein (660 kDaltons) – thyroglobulin. On sections, follicular cells vary in shape from flat to columnar, and follicles are characterized by an extremely diverse diameter. The gland is covered with a capsule of loose connective tissue, from which septa (septa) extend into the depths of the parenchyma. As the septa gradually thin out, they are transformed into thin irregularly shaped layers of connective tissue, formed mainly by reticular fibers, which reach all the follicles, separating them from each other. The thyroid gland is an organ with an extremely highly developed vascular system, an extensive network of blood and lymph capillaries surrounding the follicles. The endothelial cells of these capillaries, as in other endocrine glands, are fenestrated. This structure facilitates the transport of molecules between the cells of the gland and the blood capillaries. The morphological features of the thyroid follicles vary depending on the site of the gland and its functional activity. Inside the same gland, larger follicles filled with colloid and lined with cubic or flat epithelium are found next to the follicles that are formed by columnar epithelium. Despite such differences, if the follicular epithelium, on average, is flat, then such a gland is considered hypoactive. The thyroid gland is located on the basal lamina. Its cells have characteristics that indicate that they simultaneously synthesize, secrete, absorb and digest proteins. The basal part of these cells contains a well-developed granular endoplasmic reticulum (grEPS). The nucleus is usually round and located in the center of the cell. In the apical part are the Golgi complex, consisting of several components, and small secretory granules, the contents of which are similar to a follicular colloid. Numerous lysosomes with a diameter of 0.5-0.6 microns and individual large phagosomes are located in this area. The cell membrane of the apical pole forms a moderate number of microvilli. Mitochondria and cisterns of the granular endoplasmic reticulum (grEPS) are scattered throughout the cytoplasm. The colloid fills the cavity of the follicle, although it often separates from the cell boundary as a result of artifacts. Colloid coloration is variable – sometimes it is basophilic, sometimes acidophilic. Thyroglobulin is stained by the Schiff method with iodic acid (SIR reaction) due to its high sugar content. Another type of cells present in the thyroid gland is parafollicular, or C-cells, which are found in the follicular epithelium or form isolated groups between the thyroid follicles. Parafollicular cells are largerthan thyroid follicle cells, and under a light microscope have a lighter color. They contain a small number of cisterns of the granular endoplasmic reticulum (grEPS), elongated mitochondria, and a large Golgi complex. The most specific feature of these cells is the numerous small (100-180 nm diameter) granules containing the hormone. Parafollicular cells synthesize and secrete calcitonin, a hormone whose main effect is to lower blood calcium levels by inhibiting bone resorption. Calcitonin secretion is stimulated by an increase in the concentration of calcium in the blood.