| Biochemical actions of thyroid hormones
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| The thyroid hormones may be considered the accelerator pedal of metabolism
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| T3 and T4 increase the metabolic rate of a wide range of tissues and thereby affect the whole body, basal metabolic rate. Most, if not all, of these actions result from altered transcription brought about by the binding of T3 to its nuclear receptor (see Chapter 38).
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Thyroid hormones augment thermogenesis by increasing mitochondrial oxidative metabolism driven by the increase in ATP utilization that occurs as a result of increased sodium-potassium (Na+/K+)-dependent ATPase activity. Lipolysis is stimulated by cAMP-dependent activation of hormone sensitive lipase, thus producing fatty acids that can be oxidized to generate the ATP used for thermogenesis. Increase in both glycogenolysis and gluconeogenesis occurs to balance the increased use of glucose as a fuel for thermogenesis. The rate of synthesis of many structural proteins, enzymes, and other hormones is also affected as a result of thyroid hormone action.
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