Calorigenesis, thyroid hormones

Thyroid compounds are necessary for metabolism, growth, and development. T4 s primary action is related to calorigenesis and protein synthesis. Thyroid hormones potentiate the effects of catecholamines. About half of T4 is converted to T3. T3 is three to five times more potent than T4. 

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... 

Physiological Actions of Thyroid Hormones—Oxygen Consumption and Calorigenesis... 

Because of the role of mitochondria in cellular respiration and energy production, efforts to elucidate the mechanism of thyroid hormone action in metabolism and calorigenesis have focused on mitochondrial studies. Thyroid hormones in vitro are known to uncouple oxidative phosphorylation in isolated mitochondria, but these effects occur at unphysiological doses of T4. In physiological concentrations, T4 increases adenosine triphosphate (ATP) formation and the number and inner membrane surface area of mitochondria (21), but T4 does not reduce the efficiency of oxidative phosphorylation. Furthermore, 2,4-dinitrophenol, a classic uncoupler of oxidative phosphorylation, can neither relieve hypothyroidism nor duplicate other physiological effects of thyroid hormones. 

Effects on growth and calorigenesis are accompanied by a pervasive influence on metabolism of drugs as well as carbohydrates, fats, proteins, and vitamins. Many of these changes are dependent upon or modified by activity of other hormones. Conversely, the secretion and degradation rates of virtually all other hormones, including catecholamines, cortisol, estrogens, testosterone, and insulin, are affected by thyroid status. 

The thyroid gland synthesizes thyroxine (T4) and triiodothyronine (T3), and these hormones are involved in the regulation of growth and development, thermoregulation and calorigenesis, metabolism of carbohydrates, proteins, and lipids, and hypophyseal thyrotropin secretion (see Figure 75). 

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