Thyroid hormones carbohydrate metabolism

Triiodothyronine (3, 5,3-L-triiodothyronine, T3) is a thyroid hormone. It is producedby outer ring deiodination of thyroxine (T4) in peripheral tissues. The biologic activity of T3 is 3-8 times higher than that of T4. T3 is 99.7% protein-bound and is effective in its free non-protein-bound form. The half-life of triiodothyronine is about 19 h. The daily tur nover of T3 is 75%. Triiodothyronine acts via nuclear receptor binding with subsequent induction of protein synthesis. Effects of thyroid hormones are apparent in almost all organ systems. They include effects on the basal metabolic rate and the metabolisms of proteins, lipids and carbohydrates. 

Pharmacology Thyroid hormones enhance oxygen consumption by most tissues of the body and increase the basal metabolic rate and metabolism of carbohydrates, lipids, and proteins in the body. 

There is no discrete target tissue for thyroid hormones virtually every cell in the body is affected by thyroid hormones in some way. These hormones are intimately involved in the maintenance of normal function in virtually every cell type, including cellular responsiveness to other hormones, to the availability of metabolic substrates, to growth factors, and so on. Thyroid dysfunction can produce dramatic changes in the metabolism of proteins, carbohydrates, and lipids at the cellular level that can have repercussions for the operation of the cardiovascular, gastrointestinal, musculoskeletal, reproductive, and nervous systems. Some of the clinical manifestations of thyroid dysfunction are presented next in the discussions of hypothyroid and hyperthyroid states. 

Metabolic Effects. Thyroid hormones affect energy substrate utilization in a number of ways. For instance, these hormones increase intestinal glucose absorption and increase the activity of several enzymes involved in carbohydrate metabolism. Thyroid hormones enhance lipolysis by increasing the response of fat cells to other lipolytic hormones. In general, these and other metabolic effects help to increase the availability of glucose and lipids for increased cellular activity. 

Q3 The thyroid hormones thyroxine and triiodothyronine have many metabolic effects. In adults they increase metabolic rate, oxygen and calorie consumption, stimulate carbohydrate metabolism and turnover of protein, deplete fat stores and increase catabolism of free fatty acids. Thyroid hormones stimulate heart rate and force and increase pulmonary ventilation, gastrointestinal motility and central nervous system (CNS) activity. Actions on the heart can result in an increased incidence of dysrhythmias. Thyroid hormones are critical for the normal growth and development of the infant, particularly in respect of skeletal growth and maturation of the CNS. 

Thyroid hormones are necessary for metabolism, growth, and development. The main effect of thyroid hormones is increased metabolic rate, increased oxygen consumption, and increased metabolism of carbohydrates. Because the mixture contains both T3 and T4, systemic toxicity will be evident within a few hours and may be quite prolonged. Synthetic products that contain only T4 can have a latent period of several days before the development of significant symptoms. 

The answer is c. (Murray, pp 627-661. Scriver, pp 3897-3964. Sack, pp 121-138. Wilson, pp 287-320.) Certain amino acids and lipids are dietary necessities because humans cannot synthesize them. The energy usually obtained from carbohydrates can be obtained from lipids and the conversion of some amino acids to intermediates of the citric acid cycle. These alternative substrates can thus provide fuel for oxidation and energy plus reducing equivalents for biosynthesis. Iodine is important for thyroid hormone synthesis, while calcium is essential for muscle contraction and bone metabolism. 

An increased secretion of thyroid hormone results in an overall increase in metabolic rate in all tissues. There is a general increase in metabolism of carbohydrates, fats and proteins together with an increase in oxygen consumption and heat production. 

Thyroid hormone effects on metabolism arc diverse. The rates of protein and carbohydrate synthesis and catabolism are inlluenced. An example of the effect of thyroid hormones on lipid metabolism is the observation of a high serum cholesterol in some hypothyroid patients. This is a consequence of a reduction in cholesterol metabolism due to down regulation of low-density lipoprotein (LDL) receptors on liver cell membranes, with a subsequent failure of sterol excretion via the gut. 

Levothyroxine is indicated in the treatment of hypothyroidism (0.05 mg initially) of myxedema coma (0.4 mg IV initially) of thyroid-stimulating hormone (TSH) suppression in thyroid cancer of euthyroid and nodules goiter and of thyroid suppression therapy (2.6 mcg/kg/day for 7 to 10 days). Levothyroxine increases the metabolic rate of tissue. It affects protein and carbohydrate metabolism, promotes gluconeogenesis, increases the utilization and mobilization of glycogen stores, stimulates protein synthesis, and regulates cell growth and differentiation. The orally administered... 

Uncontrolled insulin-dependent diabetes is accompanied not only by defective carbohydrate metabolism, which results in hyperglycemia, h3q)erlipidemia, and hyperketonemia, but it is also associated with hormonal perturbation, including a reduction in circulating testosterone, thyroid hormone, and plasma Gtf . As described earlier in this chapter, these hormones regulate many liver P450 enzymes, either directly or indirectly. Accordingly, the diabetic state is... 

Effects of thyroid hormone The organ level actions of the thyroid drugs include normal growth and development of the nervous, skeletal, and reproductive systems and control of metabolism of fats, carbohydrates, proteins, and vitamins. The results of excess thyroid activity (thyrotoxicosis) and hypothyroidism (myxedema) are summarized in Table 38-1. 

The thyroid hormones, T4 and T3, play numerous, profound roles in regulating metabolism, growth, and development and in maintaining homeostasis. Their reactions and products influence carbohydrate metabolism, protein synthesis and breakdown, and cardiovascular, renal, and brain function. It generally is believed that these actions result from effects of thyroid hormones on protein synthesis. 

Thyroid hormones regulate the turnover of carbohydrates, lipids, and proteins. They promote glucose absorption, hepatic and renal gluconeogenesis, hepatic glycogenolysis, and glucose utilization in muscle and adipose tissue (18). They increase de novo cholesterol synthesis but increase low-density lipoprotein degradation and cholesterol disposal even more, leading to a net decrease in total and in low-density lipoprotein cholesterol plasma levels (19). Thyroid hormones are anabolic when present at normal concentrations they then stimulate the expression of many key enzymes of metabolism. ... 

Muller MJ, Seitz HJ. Thyroid hormone action on intermediary metabolism. Part I respiration, thermogenesis, and carbohydrate metabolism. Klin Wochenschr 1984 62 11-18. 

Thyroid hormones act on tissues all over the body and have far-reaching effects on energy metabolism, turnover of proteins, lipids and carbohydrates and consequently growth. Thyroid deficiency in early childhood, if untreated, leads to stunted growth and poor development of the nervous system. Such individuals are termed cretins. This is a word that has unfortunately passed into common usage as a term of abuse but refers to a specific clinical condition brought about by endocrine insufficiency. 

The thyroid hormones affect mahy facets of metabolism [25-32]. The metabolism of nitrogen, carbohydrates, and lipids are all markedly altered in hyper-and hypothyroidism. The biochemical sequence leading to the alteration is not known, yet these metabolic changes are of considerable importance in clinical medicine. 

Although it is generally recognized that the thyroid secretions exert a profound effect on carbohydrate metabolism in animal tissues, no clear-cut evidence is available which would implicate a member of the glycolytic system in the chain of events by which the hormone exerts its effect on the organism. 

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