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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 trachea (windpipe) and connected by a narrow band of tissue; in normal adults the thyroid gland weighs 10 to 15 grams (0.4 to 0.5 ounce). The lobes of the gland, as well as the band, consist of numerous tiny sacks called follicles. The shell of each follicle consists of a single layer of cells wrapped in a thin membrane, and each follicle is surrounded by many blood capillaries. The space inside the follicle is filled with a viscous fluid called colloid, which contains mostly thyroglobulin, the storage form of thyroid hormone. There are actually two thyroid hormones, thyroxine (T4, or tetraiodothyronine) and triiodothyronine (T3). Their structure is identical, except that thyroxine has four iodine atoms and triiodothyronine has three. Under normal conditions, the thyroid produces more thyroxine than triiodothyronine. The raw materials needed to form thyroid hormone are iodine and the amino acid tyrosine, both of which are normally found in a persons diet. The iodine is actively taken up from the plasma by thyroid cells, in which it is incorporated into tyrosine molecules within molecules of thyroglobulin. The two hormones are formed by condensation of two iodinated tyrosine molecules within the thyroglobulin, and the thyroglobulin and incorporated thyroid …

Tags: Disease, Gland, Problems, Thyroid, تروئید, غدۀ