thyroid gland

a two-lobed endocrine gland, located at the base of the neck that secretes two hormones that regulate the rates of metabolism, growth, and development. Cf. thyroxine, triiodothyronine.
[1685-95]

* * *

Endocrine gland in the throat that secretes hormones vital to metabolism and growth.

Secretion of thyroid hormones
mostly thyroxine (T4)
is controlled by thyroid-stimulating hormone (TSH), released by the pituitary gland when the level of thyroid hormones in the blood drops below a certain threshold (see endocrine system). These hormones' primary action in adults is to regulate cellular oxygen consumption (metabolic rate). They also lower blood cholesterol and are necessary for normal growth and development in children. The thyroid also produces calcitonin, a hormone that stimulates deposition of calcium from the blood into the bones, balancing the action of parathyroid hormone. See also goitre; Graves disease; iodine deficiency.

* * *

 endocrine (endocrine system, human) 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 person's 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 hormone are stored in the colloid. The thyroglobulin is reabsorbed into the thyroid cells and then broken down into its constituent parts; this frees thyroxine and triiodothyronine for release into the bloodstream, to be distributed throughout the body.

      The two thyroid hormones have many actions. One of the most important is to regulate the metabolism of nutrients and the consumption of oxygen in cells (i.e., the metabolic rate of tissues). The hormones also stimulate the contraction of heart muscle and increase heart rate, stimulate nerve function, and increase the utilization of cholesterol and other nutrients. They are also necessary for normal growth and brain development in fetuses and infants; in both the unborn and the newborn, thyroid deficiency is associated with dwarfism and mental retardation.

      The synthesis and secretion of thyroxine and triiodothyronine are regulated by thyrotropin (thyroid-stimulating hormone, or TSH), which is produced in the anterior pituitary gland. Under normal conditions, the level of thyroid hormone production is closely controlled by thyrotropin, and vice versa. Therefore, small increases or decreases in thyroid hormone secretion lead to decreases or increases, respectively, in thyrotropin secretion, so that the level of thyroid hormone in the blood serum (serum) is kept within a narrow range.

      The most common thyroid disease is thyroid nodular disease (the appearance of small, usually benign lumps within an otherwise healthy gland), followed by hypothyroidism (production of too little thyroid hormone), hyperthyroidism (production of excess thyroid hormone), and thyroid cancer.

 The thyroid gland is also the site of the production of calcitonin, a hormone that can lower serum calcium concentrations. The cells that produce calcitonin, which are called C cells or parafollicular cells, arise separately from the thyroid and migrate into it during development of the embryo. (In some animals the C cells remain separate from the thyroid.) Calcitonin is secreted in response to high serum calcium concentrations, and it lowers the concentrations acutely by inhibiting the resorption of bone. However, its action wanes within days, so calcitonin therapy is not an effective treatment for high calcium levels.
 

* * *


Universalium. 2010.

Look at other dictionaries:

  • thyroid gland — n THYROID (1) * * * a large endocrine gland situated in the base of the neck. It consists of two lobes, one on either side of the trachea, that are joined by an isthmus (sometimes a third lobe extends upwards from the isthmus). The thyroid gland… …   Medical dictionary

  • Thyroid gland — Thyroid Thy roid, a. [Gr. qyreoeidh s shield shaped; qyreo s a large, oblong shield (from qy ra a door) + e i^dos form: cf. F. thyro[ i]de, thyr[ e]o[ i]de.] 1. Shaped like an oblong shield; shield shaped; as, the thyroid cartilage. [1913… …   The Collaborative International Dictionary of English

  • thyroid gland — thyroid UK [ˈθaɪrɔɪd] / US [ˈθaɪˌrɔɪd] or thyroid gland UK / US noun [countable] Word forms thyroid : singular thyroid plural thyroids Word forms thyroid gland : singular thyroid gland plural thyroid glands medical an organ in your neck that… …   English dictionary

  • thyroid gland — a large endocrine gland situated in the base of the neck. It consists of two lobes, one on either side of the trachea, that are joined by an isthmus (sometimes a third lobe extends upwards from the isthmus). The thyroid gland consists of a large… …   The new mediacal dictionary

  • thyroid gland — gland that creates a hormone that increases the exchange of the materials in the body …   English contemporary dictionary

  • thyroid gland — noun located near the base of the neck (Freq. 6) • Syn: ↑thyroid • Derivationally related forms: ↑thyroidal (for: ↑thyroid) • Hypernyms: ↑endocrine …   Useful english dictionary

  • thyroid gland — A gland located beneath the voice box (larynx) that produces thyroid hormone. The thyroid helps regulate growth and metabolism …   English dictionary of cancer terms

  • thyroid gland — noun A large butterfly shaped endocrine gland situated on the front of the neck that produces various hormones. Removal due to thyroid cancer is termed a total thyroidectomy …   Wiktionary

  • thyroid gland — thy′roid gland n. anat. a two lobed endocrine gland at the base of the neck and secreting two hormones that regulate the rates of metabolism, growth, and development Compare thyroxine triiodothyronine …   From formal English to slang

  • thyroid gland — /ˈθaɪrɔɪd glænd / (say thuyroyd gland) noun a bilobate ductless gland lying on either side of the windpipe or trachea and connected below the larynx by a thin isthmus of tissue. Its internal secretion is important in regulating the rate of… …   Australian English dictionary


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.