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The thyroid gland synthesizes the hormones thyroxine (T4) and triiodothyronine (T3), iodine-containing amino acids that regulate the body’s metabolic rate. Adequate levels of thyroid hormone are necessary in infants for normal development of the CNS, in children for normal skeletal growth and maturation, and in adults for normal function of multiple organ systems. Thyroid dysfunction is one of the most common endocrine disorders encountered in clinical practice. Although abnormally high or low levels of thyroid hormones may be tolerated for long periods of time, usually there are symptoms and signs of overt thyroid dysfunction.
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Normal Structure & Function
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The normal thyroid gland is a firm, reddish brown, smooth gland consisting of two lateral lobes and a connecting central isthmus (Figure 20–1). A pyramidal lobe of variable size may extend upward from the isthmus. The normal weight of the thyroid ranges from 30–40 g. It is surrounded by an adherent fibrous capsule from which multiple fibrous projections extend deeply into its structure, dividing it into many small lobules. The thyroid is highly vascular and has one of the highest rates of blood flow per gram of tissue of any organ.
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Histologically, the thyroid gland consists of many closely packed acini, called follicles, each surrounded by capillaries and stroma. Each follicle is roughly spherical, lined by a single layer of cuboidal epithelial cells and filled with colloid, a proteinaceous material composed mainly of thyroglobulin and stored thyroid hormones. When the gland is inactive, the follicles are large, the lining cells are flat, and the colloid is abundant. When the gland is active, the follicles are small, the lining cells are cuboidal or columnar, the colloid is scanty, and its edges are scalloped, forming reabsorption lacunae (Figure 20–2). Scattered between follicles are the parafollicular cells (C cells), which secrete calcitonin, a hormone that inhibits bone resorption and lowers the plasma calcium level (see Chapter 17).
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The ultrastructure of a follicular epithelial cell is diagrammed in Figure 20–3. The cells vary in appearance with the degree of gland activity. The follicular cell rests on ...