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The thyroid gland is a fair-sized endocrine organ (25–40 g) that has two lobes, one on each side of the trachea below the larynx. These two lobes are connected by a bridge (isthmus) located in front of the trachea. The thyroid is composed of small cystlike units of less than one millimetre in diameter. These units (follicles) are composed of a simple cuboidal epithelium surrounding a cavity containing a semi-gelatinous colloid. The epithelium sits on a continuous basement membrane separating the follicular cells from a small amount of connective tissue containing small vessels (top drawing). The follicular cells secrete colloid containing thyroglobulin, a glycoprotein, whose components are thyroxine and diiodothyronine, the thyroid hormones. Thus, the colloid is a storage site for thyroid hormones. Stimulated by TSH, the follicular cells incorporate by phagocytosis small quantities of colloid; the colloid droplets thus formed acquire hydrolytic enzymes by fusion with Golgi-derived lysosomes (bottom drawing). The hydrolyzed thyroglobulin yields the thyroid hormones which diffuse through the basal plasma membrane of the follicular cells and reach the fenestrated capillaries and the blood circulation. These hormones regulate cellular metabolism.
In addition to follicular cells, some follicles are associated with a second cell type, the parafollicular or clear cells (C cells) (top drawing). These cells have an ovoid nucleus and a lightly stained large cytoplasm. These cells are located inside the basement membrane of the follicles but are always separated from the colloid by the cytoplasm of follicular cells. The C cells secrete calcitonin, which regulates the concentration of calcium in tissues. The C cells also secrete somatostatin.
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