Thyroid hormones definition

Tyrosine is a (crystalline) amino add with the formula HOQH4CH2CH(NH2) COOH. It results from the hydrolysis of proteins, that is, the reaction of proteins with water. The -C6H4- group within the compoimd denotes a ring-type stmcture. Another definition provided is that tyrosine is found in most proteins and can be synthesized metabolically from phenylalanine. Furthermore, tyrosine is a precursor for thyroid hormones, melanin (the dark pigment of body parts, notably, the skin), and catecholamines (that affect the sympathetic nervous system). 

In the absence of definitive evidence that iodine loss through excessive sweating can induce a relative iodine deficiency with consequences for thyroid hormone formation, there is not as yet a case for iodine supplementation of those involved in vigorous exercise. However, the... 

Behavioral disorders, including hyperactivity and impaited concentration, are known to be associated with hyperthyroidism and hypothyroidism, respectively. This evidence validated the hypothesis that ADHD might be similarly related to thyroid disease. Nonetheless, the vast majority of the studies carried out to address this issue failed to demonstrate a definite association between ADHD and thyroid function abnormalities, and therefore the effective role of thyroid hormones in the pathogenesis of the disorder became a candidate for reassessment. It must be pointed out however, that most of these studies evaluated thyroid function in schoolchildren or adults, without taking into account any previous thyroid dysfunction suffered either by them or their mothers during gestation. 

PTU and MMI reduce thyroid hormone synthesis by inhibiting the oxidation and organic binding of thyroid iodide (Cooper, 1986, 1998, 2005 Abraham et al, 2005 Azizi, 2006 Pearce and Braverman, 2004). These medications are not curative. Rather, they palliate the hyperthyroid state until it spontaneously resolves or definitive treatment is rendered. 

There is definitely an overlap in responses obtained in normd subjects and those obtained in patients with partial thyroid deficiency, even if the number of injections of TSH is reduced. However, failure of the thyroid uptake and total T4 to increase after TSH is strong evidence of primary thyroid disease. In the patients receiving thyroid hormone therapy, a significant response to TSH fairly conclusively excludes underlying thyroid failure unless the patient has been taking this therapy in full doses for many years. The test is probably most useful in distinguishing idiopathic thyroid failure from secondary or pituitary hypothyroidism. 

So far no definite therapeutic applications have been described for TRH. It has been suggested that it has antidepressant effects, but further controlled studies (M15) did not substantiate the initial reports. The role of TRH as a tool in the treatment of thyroid carcinoma (by increasing uptake of the therapeutic dose of radioiodine by the tumor) (F4) is as yet not confirmed. TRH could be used in the treatment of hypothalamic (tertiary) hypothyroidism, but it does not offer any advantage over thyroid hormone replacement. We have not observed any beneficial effect of TRH in regenerating thyroid remnants after surgery. It has been reported that TRH administration to cows results in an elevation of PRL which increases milk production (K3). Further controlled studies of the applications of TRH in the dairy industry are required. 

This chapter deals largely with the action of thyroid hormones on bile acid metabolism, because few definitive studies with other hormones (e.g., pituitary, adrenal, gonadal) have been carried out so far. 

On the other hand, lack of thyroid hormone due to deficient iodine intake can be a contributory cause for regional high incidences of atherosclerosis such as was demonstrated by Uotila el al. (1958), Roine et al. (1958), and Keys et al. (1958) in Finland. Toxemia of pregnancy is certainly not infrequent (Karvonen, 1958a) here, but definite data are not yet at hand. 

By definition all hormones affect the behavior of their target cells. Examples of the interplay between endocrine disturbances and their biochemical consequences are provided by some of the diseases of the thyroid, which directly affects basal metabolic rate, and diabetes mellitus, where glucose metabolism is deranged. 

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