The Probert Encyclopaedia of Medicine

THYROID

The thyroid is an endocrine gland near the larynx in vertebrates. It influences growth and development. The thyroid gland in human beings is a brownish-red organ having two lobes connected by an isthmus; it normally weighs about 28 g 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 (T4), and triiodothyronine (T3). 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 controlled by the thyroid- stimulating hormone (TSH) of the pituitary gland. Pituitary TSH, in turn, is regulated by a substance called thyroid-stimulating hormone releasing factor (TRF), which is secreted by the hypothalamus. Thyroglobulin is especially rich in iodine. Although the thyroid gland constitutes about 0.5 percent of the total human body weight, it holds about 25 percent of the total iodine in the body, which is obtained from food and water in the diet. Iodine usually circulates in the blood as an inorganic iodide and is concentrated in the thyroid to as much as 500 times the iodide level of the blood.
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