There are a wide range of environmental temperatures located all around the world, from the low -90° C to high 60° C at the extremes. When looking at the core body temperature however the range is closer together at about 36° C to 40° C. From there we living things like humans can only go so far dealing with outside temperatures. (Near the end of the article 'What environmental changes can we handle?') There are a lot of negative consequences when the core body temperature changes too much. Nature has many ways for endothermic creatures to maintain their body temperature for hot and cold environments like size and shape of the body to having better insulation, fur, sweating, etc.
My question is would it be possible to change the range of endothermic thermoregulation to something higher or lower and by how much? I figure there would be limiting factors like water's boiling and freezing point. I figure there is also an operational range for digesting food in an efficient manner when it comes to enzymes breaking down proteins, fats, and such. But otherwise I do not know if it is possible for an endothermic creature to have a core body temperature of lets say the extremes of near 0° C or 100° C.