Hormone triiodothyronine

The gland is situated in the neck across the front of the trachea. It secretes thyroxine (T4), which is converted to the active form of the hormone, triiodothyronine (T3), in peripheral tissues. It stimulates metabolic activity in tissues so that it increases heat production (for example, by stimulating protein turnover and substrate cycles). 

The hormone triiodothyronine (T3) accelerates both total energy expenditure and protein degradation. The hormone secreted by the thyroid gland is thyroxine, which is converted to the active hormone T3 in a process that removes an iodine atom from the 5 position of the thyronine ring. If, however, an iodine atom is removed from the 3 position, the result is the formation of reverse-Ts... 

The normal thyroid gland secretes sufficient amounts of the thyroid hormones—triiodothyronine (T3) and tetraiodothyronine (T4, thyroxine)—to normalize growth and development, body temperature, and energy levels. These hormones contain 59% and 65% (respectively) of iodine as an essential part of the molecule. Calcitonin, the second type of thyroid hormone, is important in the regulation of calcium metabolism and is discussed in Chapter 42. 

Three types of iodothyronine deiodinase remove iodine atoms from thyroxine to form the active thyroid hormone triiodothyronine and also to inactivate the hormone by removing additional iodine531 541-546 (see also Chapter 25). In this case the - CH2- Se- may attach the iodine atom, removing it as I+ to form -CH2-Se-I. The process could be assisted by the phenolic -OH group if it were first tautomerized (Eq. 15-60). 

Eukaryote organisms primarily respond to external signals by an initial signal perception by receptors. In general, such receptors can be either cytosolic or located on the plasma membrane [13-15]. The former mechanism applies to thyroid hormones (triiodothyronine and tetraiodothyronine or thyroxine), retinoids (e.g. retinoic acid), the insect developmental hormones such as ecdysone, steroid hormones (such as... 

FIGURE 31-1 Structure of the thyroid hormones triiodothyronine (T3] and thyroxine (T4X Addition of one iodine atom [I] to tyrosine produces monoiodotyrosine addition of a second iodine atom produces diiodotyrosine. A monoiodotyrosine and diiodotyrosine combine to form triiodothyronine (T3X Coupling of two diiodotyrosines forms thyroxine (T4X... 

Following the completion of her Ph.D. in 1939, Henley accompanied Harington on his move to the National Institute for Medical Research (NIMR) in Hampstead. Her field of specialisation was iodoproteins, and her research on them over the rest of her career was to garner her worldwide fame. The Second World War disrupted her research, as she was seconded to various war-work projects. The last of these, helping the survivors at the liberation of a concentration camp, proved so traumatic that she became a heavy smoker, the probable cause of her death 25 years later. In 1952, she isolated the thyroid hormone, triiodothyronine. The discovery of this new and highly unusual hormone contributed largely to her election as Fellow of the Royal Society only two years later. 

Q3 Make a summary of the actions of the thyroid hormones triiodothyronine (T3) and thyroxine (T4)... 

Hyperthyroidism is associated with an increase in the levels of thyroid hormone, triiodothyronine (T3) and thyroxin (T4), in the body, which in turn raises the metabolic rate of all tissues. 

OH-PCBs can also influence thyroxine metabolism. Some of the OH-PCBs that are retained in blood were shown to strongly inhibit sulfation of thyroxine in vitro [204-206]. As sulfation is a major regulation pathway of thyroxine in the fetus, the OH-PCBs may negatively influence the development of the fetus, and in particular fetal brain development [44]. Diodinase mediation is another pathway for thyroxine metabolism e.g., to the active hormone triiodothyronine. Hydroxylated metabolites of CB-77 were shown to inhibit triiodothyronine formation in an in vitro assay using rat hepatic microsomes [207],... 

Thyroid-stimulating hormone (TSH) that regulates the growth of the thyroid gland and the iodination of amino acids to produce the thyroid hormones triiodothyronine and thyroxine (see Chapter 52)d... 

Cytomel is a syntehtic form of the endogenous thyroid hormone triiodothyronine (T-3). Thyroid hormones are the primary regulators of body metabolism, and effect virtually all organ systems. T-... 

Estrogen (female sex hormone) Progesterone (steroid hormone) Triiodothyronine (thyroid hormone) Acetylaminofluorene (xenobiotic carcinogen) Meprobamate (drug for sleep)... 

Iodine is an essential trace element in the endocrine system, necessary for the production of the hormones triiodothyronine (T3) and thyroxine (T4) in the thyroid gland. Mammals thus provide the termination step for the cychng of iodine in the biosphere. This biogeochemical cycle (Figure 8.1) involves processes of oceanic release, sea-air transfer, photochemical transformation, aerosol uptake, and deposition on the land where iodine is adsorbed onto the soil and vegetation (Fuge and Johnson, 1986). 

Iodine is an essential element in humans and other mammals, which is used for the synthesis of the thyroid hormones triiodothyronine (T3) and thyroxine (T4). These hormones play a prominent role in the metabolism of most cells of the organism and in the process of early growth and development of most organs, especially brain (Anderson et al., 2000). Besides T3 and T4, reverse T3 (rT3), monoiodotyrosine (MIT), and diiodotyrosine (DIT) are also synthesized and distributed in the body of humans and animals, but only T3 and T4 have a biological function. Iodine in the human body mainly comes through dietary and water intake, and inhalation of atmospheric iodine. Due to low concentrations of iodine in the air (10—20ng/m ), food and water intake form the major source of iodine for adults, while for infants it is milk. The concentration of iodine in foodstuffs is directly related to that in the environment where the foods come from. Iodine deficiency disorders are mainly found in places where the concentration of iodine in the soil and drinking water is very low. In the water, foodsmffs, and... 

Iodine is essential for the synthesis of the thyroid hormones — triiodothyronine (T3) and thyroxine (T4). An iodine-deficient diet results directly in decreased production of thyroid hormones, which adversely affects not only brain development, but also its functions such as attention, learning and memory. In areas with iodine deficiency, besides the occurrence of more frequent thyroid nodules, the intelligence quotient (IQ) of children is much lower than that in areas with adequate iodine. 

Drugs The most common source of iodine excess in the United States is amiodarone, a widely used antiarrhythmic drug. Amiodarone contains 37% iodine and shows a structural similarity with the thyroid hormones triiodothyronine (T3) and tetraiodothyronine (T4) (Hermann, 2004 Kennedy et al, 1989 Martino et al, 2001). 

Description. Iodine, a nonmetallic trace element, is required by humans for the synthesis of thyroid hormones, triiodothyronine (Tj) and thyroxine (T4). Iodine deficiency is an important health problem throughout much of the world. Most of the Earth s iodine is found in its oceans. In general, the older an exposed soil surface, the more likely the iodine has been leached away by erosion. Mountainous regions, such as the Himalayas, the Andes, and the Alps, and flooded river valleys, such as the Ganges, are among the most severely iodine-deficient areas in the world. ° ... 

The concentration of iodine present in the animal body is very small and in the adult is usually less than 600 pg/kg. Although the element is distributed throughout the tissues and secretions, its only known role is in the synthesis of the two hormones, triiodothyronine (T3) and tetraiodothyronine (T4, thyroxine) produced in the thyroid gland (see p. 54). 

The three thioredoxin reductases act in conjunction with the sulfur protein thioredoxin and with NADPH to bind key transcription factors to DNA. The iodothyronine deiodinases modulate the thyroid hormones, helping to ensure an optimal supply of the most active member of the thyroid hormones, triiodothyronine. The different selenoprotein deiodinases are found at different sites in the body. If selenium and iodine are deficient in a human population, the thyroid deficiency is more severe (and goiters are larger) than if only iodine is lacking. This situation is endemic in some areas of central Africa, including Kivu province in the Central African Republic (formerly Zaire). 

Thyroid dysfunction is a fairly common disorder to be found in clinical practice. The thyroid synthesizes two hormones, triiodothyronine (T ) and thyroxine (T4), dipeptides containing 3 and 4 atoms of iodine respectively in each molecule. The thyroid stimulates cell metabolism of most tissues, might cause hypermetabolism symptoms loss of weight in spite of increased appetite, palpitations, tremor of the fingers, anxiety, heat intolerance. The hormone activity is mainly carried out by T3. Most of it derives by monodeiodination from T4 that is secreted directly by the thyroid. Both hormones in the blood are bound to plasma proteins. In the development of medical applications of microcalorimetry, among the first pathological conditions to be studied were thyroid dysfunctions. 

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