THYROID GLAND

endocrine gland that is situated in the throat below the larynx (voice box); the thyroid secretes hormones vital to metabolism and growth. The gland consists of two oblong lobes lying on either side of the windpipe and connected by a narrow band of tissue. The lobes of the gland, as well as the band, consist of numerous tiny sacks called follicles. The shell of each follicle is built of cells closely packed together and is wrapped in a thin membrane covered with a dense mesh of blood capillaries, which ensure a steady and abundant supply of fresh plasma through the membrane to the cells. The space inside the follicle is filled with a viscous fluid called colloid, in which thyroid hormones are stored bound to a protein called thyroglobulin. Every follicle of the thyroid gland makes thyroid hormone and secretes it into the bloodstream. Thyroid hormone consists of two closely related chemical substances, L-thyroxine (T4, or L-tetraiodothyronine) and L-triiodothyronine (T3). Under normal conditions, most of the hormone is made up of thyroxine. The raw materials necessary to form thyroid hormone are iodine and the amino acid tyrosine, both of which are normally found in a person's diet. The iodine is actively pumped out of the body fluids into the thyroid's follicle cells, which retain and concentrate it before secreting both it and thyroglobulin into the colloid. The thyroid hormones are then formed by the progressive and orderly addition of iodine atoms to tyrosine residues found on the molecules of thyroglobulin. The process takes place at the edge of the follicle cells next to the colloid. This reaction is followed by the condensation of two of the iodinated residues to form thyroxine and triiodothyronine, which are then stored in the colloid. In addition to transporting the raw materials for thyroid hormone synthesis to the colloid, the thyroid cells also remove triiodothyronine and thyroxine from the colloid and secrete these substances into the bloodstream. They do this by detaching the hormones from the thyroglobulin proteins that they are bound to. The thyroxine and triiodothyronine molecules thus freed are small enough to cross the cell wall and to enter the blood capillaries around the follicle, after which they become distributed throughout the entire body. The thyroid hormones have a number of different effects, but their primary action in adults is to regulate the oxygen consumption of cells (i.e., the metabolic rate of tissues). The hormones also lower circulating cholesterol levels, an effect for which they have been used therapeutically. They are necessary, too, for the normal growth and development of children, and their deficiency is associated with dwarfism and mental retardation. The secretion of thyroxine and triiodothyronine is under the control of thyroid-stimulating hormone (TSH), which is released from the anterior lobe of the pituitary gland. Under normal conditions, the appropriate level of thyroid hormone in the body is readily established; this induces the pituitary to stop releasing TSH. Thus, the level of thyroid hormone in the body is kept within a constant range by a mechanism in which the effects of thyroid hormone and of thyroid-stimulating hormone work in closed cycle (feedback mechanism), each hormone controlling the secretion of the other. The thyroid gland is also the site of the production of calcitonin, a hormone that buffers against undue increases in serum calcium. Hyperthyroidism is an abnormal condition characterized by the excessive secretion of thyroid hormone; it is usually accompanied by overgrowth of thyroid tissue. The most common type of hyperthyroidism is Graves' disease (q.v.).