thyroid gland
The definitions used in this glossary of terminology either have been provided by the authors of the articles, or have been extracted wholly or in part, or paraphrased from the following sources: The American Medical Association Encyclopedia of Medicine, Charles B. Clayman, MD, Medical Editor, Random House, New York, 1989; Biotechnology from A to Z, 2d Edition, William Bains, Oxford University Press, New York, New York, 2002; A Dictionary of Genetics, 6th Edition, Robert C. King and William D. Stansfield, Oxford University Press, New York, New York, 2002; Dorland's Illustrated Medical Dictionary, 29th and 30th Editions, W. B. Saunders Company, Philadelphia, 2000, 2003; Genes VII, Benjamin Lewin, Oxford University Press, New York, New York, 2000; The Gale Encyclopedia of Genetic Disorders, Volumes I and II, Stacey L. Blachford, Ed., Thomson Learning, New York, New York, 2002; The Merriam-Webster Dictionary, Merriam-Webster, Inc., Springfield, Massachusetts, 1997; Molecular Biology of the Cell, 3rd Edition, Bruce Alberts, et al., Garland Publishing, 1994; The Random House Dictionary of the English Language, Unabridged Edition, 1966; Webster's Ninth New Collegiate Dictionary, 1991.
DEFINITION:
- thyroid gland
-
An important organ of the endocrine system. The thyroid gland is situated in the front of the neck, just below the larynx (voice box). It consists of two lobes, one on each side of the trachea (windpipe), joined by a narrower portion of tissue called the isthmus.
STRUCTURE:
Thyroid tissue is composed of two types of secretory cells---follicular cells and parafollicular cells (or C cells). Follicular cells make up most of the gland. They are arranged in the form of hollow, spherical follicles, and secrete the iodine-containing hormones thyroxine (T4) and triiodothyronine (T3). The space inside the follicles is filled with a semifluid, colloid material that is essential for the production of T4 and T3.
Parafollicular cells occur singly or in small groups in the spaces between the follicles. They secrete the hormone calcitonin. Also between the follicles are numerous blood capillaries, small lymphatic vessels, and connective tissue. The entire thyroid gland is encased in a thin outer layer of connective tissue.
FUNCTION:
T4 and T3 play an important role in controlling body metabolism. Calcitonin acts in conjunction with parathyroid hormone to regulate calcium balance in the body.
DISORDERS:
The function of the thyroid gland is controlled by both the pituitary gland and the hypothalamus, so thyroid disorders may be due not only to defects in the gland itself, but also to disruption of the hypothalamic-pituitary hormonal control system. Thyroid disorders may cause symptoms due to overproduction of thyroid hormones (hyperthyroidism), underproduction of these hormones (hypothyroidism), or enlargement or distortion of the gland. Idiopathic (of unknown cause) goiter (enlargement of the gland), Graves' disease, and Hashimoto's thyroiditis are the common disorders of thyroid function. Goiter sometimes occurs without any accompanying abnormality of the thyroid function.
Other disorders include congenital defects (either a missing, underdeveloped, or maldeveloped thyroid gland), genetic disorders, infection (thyroiditis), tumors, autoimmune disorders, myxedema, hormonal disorders, nutritional disorders, and post-irradiation induced tumors.
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Used in 1 Article translation