THE THYROID GLAND
The thyroid is a butterfly-shaped gland located in the anterior aspect of the neck, inferior to the larynx (see Figure 6.10).
It covers the second to fourth tracheal rings and is surrounded by a capsule. Posteriorly, the capsule encloses the four, small parathyroid glands (discussed later). The thyroid has two lobes located on either side of the trachea. The gland becomes vis-ible when it is enlarged.
The thyroid’s main function is to maintain the me-tabolism level in the body. Therefore, the thyroid is needed to regulate lipid and carbohydrate metabo-lism and for normal growth and function. The for-mation and secretion of thyroid hormones is con-trolled by thyroid-stimulating hormone (TSH) from the pituitary. TSH levels are, in turn, regulated by the thyroid hormone levels by negative feedback mecha-nism. TSH secretion is also regulated by the releasing hormone secreted by the hypothalamus (Figure 6.6). When TSH levels are reduced in the plasma, the thy-roid gland atrophies and its function is depressed. If TSH stimulation is prolonged, the thyroid gland en-larges and hypertrophies, forming a goiter.
The principal hormones secreted by the thyroid are thyroxine (T4), triiodothyronine (T3), and calcitonin.
The hormone calcitonin is discussed below. Thyroxine and triiodothyronine are considered below.
Iodine is required for the formation of T3 and T4, and iodine from the plasma is actively absorbed by thyroid gland. The hormones are manufactured by the cells that line spherical sacs (thyroid follicles) and stored in the follicles in the form of a colloid (thyroglobulin). The thyroid gland is the only en-docrine gland that stores large quantities of hor-mones. When required, the hormones are secreted into the blood where most of them are transported bound to plasma proteins. It takes many days for thy-roid hormones to be removed from the circulation by the liver, kidneys, and other tissues. Most of the io-dine from the hormones is recycled and reused for forming the hormones.
The main hormone action is to increase oxygen consumption (increase metabolism) by most of the cells, with the exception of the brain, uterus, testis, lymph nodes, spleen, and anterior pituitary. They also affect growth and development. Thyroid hormones increase the metabolic rate by stimulating cells to use oxygen to form ATP. When ATP is produced, heat is produced and body temperature increases (calori-genic effect of the thyroid). Other effects include anincrease in protein synthesis, a breakdown of carbo-hydrates and fat, and excretion of cholesterol. In ad-dition, thyroid hormones enhance the action of adrenaline and noradrenaline. Together with other hormones, such as growth hormone and insulin, thy-roid hormones speed growth of the body, especially nervous tissue. The actions of these hormones can be better illustrated by studying individuals with hyper-thyroidism and hypothyroidism (see Abnormalitiesof Thyroid Secretion).
Calcitonin is a hormone secreted by cells that lie be-tween or outside the follicles. It causes an increase in the deposition of calcium and phosphates in bones by reducing resorption and facilitating deposition. This is accomplished by inhibiting the ac-tion of osteoclasts. As a result of the bone-building ef-fect of calcitonin, it is used to treat osteoporosis.
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