Thyroid hormone, basal metabolic rate

Wada, L., and King, J- C, (1986), Effect of low zinc intakes on basal metabolic rate, thyroid hormones, and protein utilization in adult men / Niiir. 116, 1045-1053. 

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. 

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. 

Basal metabolic rate is decreased under conditions of starvation (including dieting as noted above one more reason why it is tough to lose weight) and increased under conditions of feeding. These responses tend to keep body weight constant. Thyroid hormone increases basal metabolic rate. Hyperthyroid people tend to be slender. 

The thyroid hormone thyroxine (tetraiodo-thyronine, T4) and its active form triiodothyronine (T3) are derived from the amino acid tyrosine. The iodine atoms at positions 3 and 5 of the two phenol rings are characteristic of them. Post-translational synthesis of thyroxine takes place in the thyroid gland from tyrosine residues of the protein thyro-globulin, from which it is proteolytically cleaved before being released, iodothyronines are the only organic molecules in the animal organism that contain iodine. They increase the basal metabolic rate, partly by regulating mitochondrial ATP synthesis, in addition, they promote embryonic development. 

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. 

Mechanism of Action A synthetic form of triiodothyronine (T3), a thyroid hormone involved in normal metabolism, growth, and development. Possesses catabolic and anabolic effects. Therapeutic Effect Increases basal metabolic rate, enhances gluco-neogenesis, and stimulates protein synthesis. 

Thyroid hormones are intimately involved in regulating the basal metabolic rate. Liver tissue of animals given excess thyroxine shows an increased rate of 02 consumption and increased heat output (thermogenesis), but the ATP concentration in the tissue is normal. Different explanations have been offered for the thermogenic effect of thyroxine. One is that excess thryroxine causes uncoupling of oxidative phosphorylation in mitochondria. How could such an effect account for the observations Another explanation suggests that the thermogenesis is due to an increased rate of ATP utilization by the thyroxine-stimulated tissue. Is this a reasonable explanation Why ... 

Thyroxine and triiodothyronine have many effects, a major one in mammals and birds being stimulation of energy metabolism in tissues. It has long been recognized that a deficiency of thyroid hormone is reflected in an overall lower basal metabolic rate (Chapter 6). Maley and Lardy observed that thyroxine uncouples oxidative phosphorylation (Chapter 18) in isolated mitochondria.117 When mitochondria from animals receiving extra thyroxine were compared with those from control animals, an increased rate of electron transport was observed. However, there was little or no change in the P / O ratio. Thus, the hormone apparently increased the rate of electron transport... 

The functions of the thyroid hormones and thus of iodine are control of energy transductions. These hormones increase oxygen consumption and basal metabolic rate by accelerating reactions in nearly ail cells of the body. A part of this effect is utlribuicd lo increase in activity of many enzymes. Additionally, protein synthesis is affected by the thyroid hormones. 

Thermogenesis. T4 and T, increase the basal metabolic rate and subsequent heat production from the body, which are important in maintaining adequate body temperature during exposure to cold environments. Increased thermogenesis is achieved by thyroid hormone stimulation of cellular metabolism in... 

Quick relief can be obtained with a p-adrenoceptor blocking drug (judge dose by heart rate) though these do not block all the metabolic effects of the hormone, e.g. on the myocardium, and the basal metabolic rate is unchanged. For this reason they should not be used as sole therapy except in mild thyrotoxicosis in preparation for radioiodine treatment, and should be continued in these patients until the radioiodine has taken effect. They do not alter the course of the disease, nor biochemical tests of thyroid function. Any effect on thyroid hormonal action on peripheral tissues is clinically unimportant. It is desirable to choose a drug that is nonselective for pj and p2 receptors and lacks partial agonist effect (e.g. propranolol 20-80 mg 6-8-hourly, or timolol 5 mg once daily). Usual contraindications to P-blockade (see p. 478) should be observed, especially asthma. 

An increase in thyroid hormone levels results in an increase in the basal metabolic rate (BMR). BMR measurements can be used to assess thyroid status, as discussed in detail in Chapter 5. This method for the diagnosis of hypo- or hyperthyroidism is not in coitimon use because it is cumbersome. The increase in BMR has been associated with increases in various reactions that use ATP. The increased use of ATP is matched by an increase in activity of the respiratory chain and in Oj... 

The exact mechanism of action is not well understood. Thyroid hormones are needed for metabolism, growth, and development. Most organ systems are affected by thyroid hormones. Thyroid hormones cause an increase in the basal metabolic rate, in oxygen consumption, and in the metabolism of... 

Thyroid hormones have many important biological effects. A major function is their control of the basal metabolic rate and calorigenesis through increased oxygen consumption in tissue via the effects of thyroid hormone on membrane... 

Synthesis of thyroid hormones that regulate basal metabolic rate Protects against number of cancers... 

It is evident that variations in thyroid hormone levels are among the main physiological modulators of in vivo cellular oxidative stress. The hypermetabolic state in hyperthyroidism is associated with increases in free radical production and lipid peroxidation (LP), and the hypomet-abolic state in hypothyroidism is generally associated with a decrease in free radical production and LP in most tissues (Fernandez et ai, 1985 Venditti et ai, 1997). The development of a hyperthyroid state in vertebrates leads to enhancement of their basal metabolic rate due to an increase in the rate of O2 consumption in most tissues, excluding the spleen, testis and adult brain (Barker and Klitgaard, 1952). Thyroid hormones were shown... 

Tiredness and lethargy are the most common symptoms. Other effects include depression of the basal metabolic rate, appetite and cardiac output, Low-output heart failure may occur. The skin is dry. Thyroid hormone deprivation in early life results in irreversible mental retardation and dwarfism (cretinism), and to prevent this all newborn infants are screened and replacement therapy is given from birth. 

The thyroid hormone increases basal metabolic rate (resting energy expenditure). It has both direct actions and indirect effects on the metabolism by modifying the activities of other hormones such as catecholamines. The thyroid hormone also promotes growth. The thyroid hormone s action on metabolism is dose dependent. 

Tata, J.R. Basal metabolic rate and thyroid hormones. Advanc. Metab. Disord. 1, 153-184(1964)... 

The natural thyroid hormone, levorotatory thyroxine (LTJ, is primarily responsible for the elevation of the basal metabolic rate (BMR) as reflected in the increase in oxygen consumption. This feature of thyroxine is denoted as the calorigenic effect. Another important property of thyroxine is its influence on the heart. The intrinsic heart-rate was found to be higher in patients with hyperthyroidism and lower than normal in patients with hypoth5U oidism. It has been concluded that the differences in heart-rate between subjects with hyper- and hypothyroidism are mainly due to thyroxine acting directly on the heart and not to its potentiation of the action of catecholamines [174]. 

Iodine is a trace mineral needed by the body in only very small quantities. It is used in the synthesis of a hormone that regulates basal metabolic rate. Its deficiency results in goiter, a condition in which the thyroid gland swells as it tries to capture as many iodine particles as possible. Iodine deficiency during pregnancy can result in severe retardation of the developing baby. Iodine is readily available in iodized salt, seafood, and milk. The addition of iodine to salt has all but eliminated the effects of its deficiency in developed countries. The DRI for iodine is 150 pig/day, provided by about one-half teaspoon of iodized salt. 

Basal metabolic rate is controlled to a considerable extent by the thyroid hormone... 

Thyroid hormone activity can be determined indirectly as the basal metabolic rate (q.v.). 

Thyroxin (tetraiodothyronine) is circulated in the blood in temporary union with blood proteins hence, what the chemical test actually measures is the protein-bound iodine content of the blood, which, in turn, serves as an index of relative activity of the thyroid in releasing its hormone into the blood—hence, the approximate basal metabolic rate. 

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