Thyroid Gland
Thyroid Gland, endocrine gland found in almost all vertebrate animals and so called because it is located in front of and on each side of the thyroid cartilage of the larynx (see Endocrine System ). It secretes a hormone that controls metabolism and growth. The thyroid gland in human beings is a brownish-red organ having two lobes connected by an isthmus; it normally weighs about 28 g (about 1 oz) and consists of cuboidal epithelial cells arranged to form small sacs known as vesicles or follicles. The vesicles are supported by connective tissue that forms a framework for the entire gland. In the normal thyroid gland, the vesicles are usually filled with a colloid substance containing the protein thyroglobulin in combination with the two thyroid hormones thyroxine , also called tetraiodothyronine, and triiodothyronine. These hormones are composed of the amino acid tyrosine, containing four and three iodine atoms, respectively. The amount of thyroglobulin secreted by the thyroid is cont