Thyroid Gland Anatomy Physiology and Histology

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Adrenal Gland : • Adrenal Gland | Anatomy and Physiology • Pituitary Gland : • Pituitary Gland | Hormones and Hypoth... • Endocrine System : • Endocrine System | Introduction • The thyroid gland is the largest adult gland to have a purely endocrine function, weighing about 25 g. It lies adjacent to the trachea immediately below the larynx, and is named for the nearby shield- like thyroid15 cartilage of the larynx. It is shaped like a butterfly wrapped around the trachea, with two winglike lobes usually joined inferiorly by a narrow bridge of tissue, the isthmus . • Histologically, the thyroid is composed mostly of sacs called thyroid follicles Each is filled with a protein-rich colloid and lined by a simple cuboidal epithelium of follicular cells. These cells secrete about 80 micrograms (μg) of thyroid hormone (TH) daily. About 90% of this is in a form called thyroxine, T4, or tetraiodothyronine • The other 10% is a form with three iodine atoms, called triiodothyronine or T3. That is, the expression thyroid hormone refers to T3 and T4 collectively. • Like other endocrine glands, the thyroid releases these hormones directly into the bloodstream. Each follicle is surrounded by a basketlike network of capillaries, the globular clusters of blood vessels seen in this chapter’s opening photo. These are supplied by the superior and inferior thyroid arteries seen in figure 17.9a. The thyroid receives one of the body’s highest rates of blood flow per gram of tissue and consequently has a dark reddish brown color. • Thyroid hormone is secreted or inhibited in response to fluctuations in metabolic rate. The brain monitors the body’s metabolic rate and stimulates TH secretion through the action of TRH and TSH as depicted in figure 17.7. The primary effect of TH is to in- crease one’s metabolic rate. As a result, it raises oxygen consumption and has a calorigenic effect—it increases heat production. To ensure an adequate blood and oxygen supply to meet this increased metabolic demand, thyroid hormone also raises the respiratory rate, heart rate, and strength of the heartbeat. It stimulates the appetite and accelerates the breakdown of carbohydrates, fats, and protein for fuel. Thyroid hormone also promotes alertness and quicker re- flexes; growth hormone secretion; growth of the bones, skin, hair, nails, and teeth; and development of the fetal nervous system. • The thyroid gland also contains nests of parafollicular cells, also called clear (C) cells, at the periphery of the follicles. They respond to rising levels of blood calcium by secreting the hor- mone calcitonin. Calcitonin antagonizes parathyroid hormone (discussed shortly) and stimulates osteoblast activity, thus promot- ing calcium deposition and bone formation. It is important mainly

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