Thyroid Hormone and the Energetic Cost of Keeping Body Temperature

By definition, homeothermic species maintain their core temperature (Tc) regu-lated within narrow limits. In most of these species, the hypothalamic thermostat is set somewhere between 36 and 38C, and Tc is tightly kept there, in spite of highly variable ambient temperatures. Several mechanisms to maintain body temperature have been selected during evolution. Vasoconstriction and piloerec-tion are mechanisms to save heat, while vasodilatation, sweating and perspiration are mechanisms to rapidly dissipate heat. To maintain temperature in environ-ments usually colder than Tc, homeothermic species need to produce more heat than poikilothermic species. Energy transformations generate heat simply by virtue of the laws of thermodynamics and so the energy transformations inherent to life generate heat. Such heat, generated as byproduct of cell vital functions, is called obligatory thermogenesis (OT) and is expectedly higher in homeothermic species, as a consequence of which metabolic rate is substantially higher in these species than in poikilothermic species (Else and Hulbert, 1981). In addition to having a higher OT, homeothermic species can also activate specific mechanisms