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A novel syndrome combining thyroid and neurological abnormalities is associated with mutations in a monocarboxylate transporter gene. Am J Hum Genet. Thyroid hormone acts directly on growth plate chondrocytes to promote hypertrophic differentiation and inhibit clonal expansion and cell proliferation. Thyroid hormones promote chondrocyte differentiation in mouse ATDC5 cells and stimulate endochondral ossification in fetal mouse tibias through iodothyronine deiodinases in the growth plate.
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J Orthop Res. Triiodothyronine induces colla-genase-3 and gelatinase B expression in murine osteoblasts. Am J Physiol. Thyroid hormones regulate hypertrophic chondrocyte differentiation and expression of parathyroid hormone-related peptide and its receptor during endochondral bone formation.
T3 affects expression of collagen I and collagen cross-linking in bone cell cultures. Biochem Biophys Res Commun. Banovac K, Koren E. Triiodothyronine stimulates the release of membrane-bound alkaline phosphatase in osteoblastic cells. Thyroid hormone activates fibroblast growth factor receptor-1 in bone. The complex building process begins with a long chain of linked tyrosine amino acids called thyroglobulin. One or two iodine atoms are first added to each tyrosine.
Then, pairs of the iodinated amino acids are clipped from the thyroglobulin chain creating mainly thyroxine and some triiodothyronine. Thyroxine, the most abundant thyroid hormone, contains four iodine atoms and is nicknamed T4, while triiodothyronine, the most active form, has three and is referred to as T3. Once made, the thyroid gland releases the hormones into the bloodstream where protein chaperones, called thyroid transport proteins, accompany them to target cells in tissues all over the body.
In the target cells, enzymes remove one of thyroxine's four iodine atoms converting the hormone into the highly active triiodothyronine. Transformation of thyroxin in body tissues is the main source of nearly all of the needed triiodothyronine as well as other lesser used thyroid hormones.
These other types include: triac 3,5,3'-triiodothyroacetic acid , tetrac 3,5,3',5' tetraiodothroacetic acid and 3,3',5' - triiodothyronine rT3. In humans, the liver, kidney, and muscle contain the most thyroid hormone.
Among animals, all vertebrates and even some invertebrates, such as sea squirts, make thyroid hormones even though invertebrates lack a thyroid gland. Thyroid hormones produce effects by docking with protein receptors in thyroid-sensitive tissues.
The hormones can bind with receptors on the cell's membrane surface and inside the cell on the mitochondria or in the nucleus. Binding activates chemical processes and protein producing genes that control a cell's energy and metabolic functions Cato et al. Thyroid hormones usually take many hours to days to achieve their final effects. Almost all body functions carried out in nearly every tissue rely on thyroid hormones.
Their actions and influence are so wide-ranging that vertebrates cannot live without them. Among other things, thyroid hormones specifically affect brain development; heart rate; lung function; blood function; bone growth; steroid hormone production and breakdown; sugar, fat, and protein breakdown; and some immune processes.
Whether alone or in concert with other hormones, the indispensable messengers regulate life-sustaining processes essential for normal growth, development, reproduction, and even behavior. For example, sufficient concentrations of thyroid hormone must be present before birth and in early life for normal brain development, bone maturation, and production of correct amounts of growth hormone.
Deficiencies can stunt growth and impair hearing, motor control, and intelligence in newborns and the young. Throughout life, thyroid hormones influence vertebrate energy demands by stimulating cells to burn more oxygen. A cell's oxygen use creates heat to moderate body temperature in warm-blooded animals and fuel metabolism the body's ability to breakdown, store, and reclaim food releasing chemical energy in the process. Less than normal concentrations of thyroid hormone impairs oxygen and energy use while higher than normal concentrations over stimulates them.
Seasonal changes such as fur molting in mammals or skin shedding by reptiles is directed by thyroid hormones. It is the inactive form and most of it is converted to an active form called triiodothyronine by organs such as the liver and kidneys. The production and release of thyroid hormones, thyroxine and triiodothyronine, is controlled by a feedback loop system that involves the hypothalamus in the brain and the pituitary and thyroid glands.
The hypothalamus secretes thyrotropin-releasing hormone which, in turn, stimulates the pituitary gland to produce thyroid stimulating hormone. This hormone stimulates the production of the thyroid hormones, thyroxine and triiodothyronine, by the thyroid gland. This hormone production system is regulated by a feedback loop so that when the levels of the thyroid hormones thyroxine and triiodothyronine increase, they prevent the release of both thyrotropin-releasing hormone and thyroid stimulating hormone.
This system allows the body to maintain a constant level of thyroid hormones in the body. The release of too much thyroxine in the bloodstream is known as thyrotoxicosis. This may be caused by overactivity of the thyroid gland hyperthyroidism , as in Graves' disease , inflammation of the thyroid or a benign tumour. Thyrotoxicosis can be recognised by a goitre , which is a swelling of the neck due to enlargement of the thyroid gland. Too little production of thyroxine by the thyroid gland is known as hypothyroidism.
It may be caused by autoimmune diseases, poor iodine intake or caused by the use of certain drugs. Sometimes, the cause is unknown.
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