The thyroid gland synthesizes the hormones thyroxine (T4) (prohormone) and triiodothyronine (T3) (active hormone), iodine-containing amino acids that regulate the body’s metabolic rate. Adequate levels of thyroid hormone are necessary in infants for normal central nervous system (CNS) development, in children for normal skeletal growth and maturation, and in adults for the 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, there are usually symptoms and signs of overt thyroid dysfunction.
NORMAL STRUCTURE & FUNCTION
The normal thyroid gland is a firm, reddish brown, smooth gland consisting of two lateral lobes and a connecting central isthmus, located in front of the trachea (Figure 20–1). A pyramidal lobe of variable size may extend upward from the isthmus. The normal weight of the thyroid ranges from 10 g to 20 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.
The human thyroid. (Redrawn, with permission, from Barrett KE et al, eds. Ganong’s Review of Medical Physiology, 25th ed. McGraw-Hill, 2016.)
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 though its function is usually not significant in the regulation of normal calcium homeostasis (see Chapter 17).
Normal and abnormal thyroid histology. (Redrawn, with permission, from Barrett KE et al, eds. Ganong’s Review of Medical Physiology, 25th ed. McGraw-Hill, 2016; Chandrasoma P et al, eds. Concise Pathology, 3rd ed. Originally published by Appleton & Lange. Copyright © by The McGraw-Hill Companies, Inc.; Gardner DG et al, eds. Greenspan’s Basic and Clinical Endocrinology, 10th ed. McGraw-Hill, 2017.)