Heat is a byproduct of the energy transformation in its various forms during the synthesis and utilization of ATP. In homeothermic animals the heat derived from the biological functions - at rest and at room temperature - is used to keep the body temperature close to 37°C. This obligatory thermogenesis is associated to the intrinsic mitochondrial thermodynamic inefficiency caused by the presence of uncoupling proteins (UCPs). During cold exposure the body is capable of generating more heat by means of the facultative thermogenesis, a series of processes that also involves the function of UCPs. The thyroid hormones influence directly the expression of UCP-1 and indirectly the expression of UCP-2 and UCP-3. Furthermore, they also accelerate the turnover of several cyclical biochemical reactions and pathways that lead to greater ATP breakdown and energy expenditure.
Thyroid hormones; Thermogenesis; UCP; ATP