Thyroid Gland

Thyroid Gland

The thyroid gland is located just below the larynx on either side and in front of the trachea ( FIGURE 16-14). It consists of two large lobes that are connected by a broad isthmus, forming a “butterfly” shape and is covered by a capsule of connective tissue with secre-tory parts called follicles. The thyroid’s lumen or central cavity is filled with a clear, amber-colored substance called colloid, which stores ­hormones the ­glycoprotein called thyroglobulin­. From this substance, thyroid hormone is synthesized, which affects nearly every cell in the body. ­Thyroid hor-mone production requires the presence of iodine and calcium. This hormone is the body’s primary metabolic hormone containing ­thyroxine, which is also known as tetraiodothyronine or T₄ and ­triiodothyronine or T₃. The number of each of these iodine -containing amine hormones indicates the number of atoms of iodine. T₃ is five times as potent as T₄. Thyroxine regulates metabolism of carbohydrates, lipids, and proteins, determining the body’s basal metabolic rate. Thyroid hormones are required for growth, development, and matura-tion of the nervous system. Thyroxine is the major hormone of the thyroid follicles. Most triiodothy-ronine forms at target tissues because of thyroxine. Most molecules of the thyroid hormones attach to thyroid- binding globulins upon entering the blood-stream. In the circulation, most remaining mole-cules attach to transthyretin, which is also called thyroid-binding prealbumin or to albumin itself.

Thyroid hormones enter target cells binding to intracellular receptors in the cell’s nucleus. They con-trol transcription of mRNA needed for protein syn-thesis. The many effects of thyroid hormones include increase of basal metabolic rate, increase of body heat production or the calorigenic effect, regulation of tissue growth and development, and maintenance of blood pressure.

Iodine salts or iodides are needed by the follicular cells to secrete thyroid hormones. Thyroid hormone secretion is controlled by the hypothalamus and pitu-itary gland. Calcitonin is another hormone secreted by the thyroid, although it is produced by its parafol-licular cells, also known as C cells, rather than the fol-licular cells. These cells lie in the follicular epithelium, protruding into the connective tissue surrounding the thyroid follicles. Calcitonin is a polypeptide hormone that regulates concentrations of blood calcium and phosphate ions and its release is controlled by blood concentration of calcium ions. Calcitonin inhibits osteoclast activity, which inhibits bone resorption and release of calcium from the bone matrix. It also stimulates calcium uptake and incorporation into the bone matrix. TABLE 16-4 discusses the actions of thy-roid hormones.