Well, today I have a fairly simple question. Is it possible for a human to have some sort of birth defect, mutation, disease or something that causes the human body to be unable to produce body heat?
Nothing that simple. A lot of physiological mechanisms in human body depend on the temperature. Blood composition, immune system, energy transfer. Exceeding the temperature by 3 degrees causes effects like hallucinations and extreme weakness (fever).
Mere lack of thermoregulation would be lethal even in early infancy stage.
The changes would need to be way more thorough to be survivable, including a whole lot of physiological processes and fundamental construction of the cells. Not something that can happen through birth defect, a simple mutation or disease. Several thousand years of evolution maybe, though I don't know what factors would encourage such a change.
To compare: can you power a PC from a solar panel? Yes, with a buffer accumulator, inverter, charger circuitry etc; lots of "backend". If you just draw wires from the panel to the power socket, it won't work.
The normal range for human body temperature, as measured with a thermometer stuck to a person's a... rmpit, is between 36.5 and 37.5 degrees Celsius, or 97.7 o 99.5 F.
Hypothermia is reduced body temperature that happens when a body dissipates more heat than it absorbs. In humans, it is defined as a body core temperature below 35.0 °C (95.0 °F). Symptoms depend on the temperature. In mild hypothermia there is shivering and mental confusion. In moderate hypothermia shivering stops and confusion increases. In severe hypothermia, there may be paradoxical undressing, in which a person removes his or her clothing, as well as an increased risk of the heart stopping.
I swear I'm not making this thing about undressing up.
You see, we mammals evolved self thermorregulation millions of years ago. Being able to heat up on metabolism alone didn't just give us an immediate advantage over cold-blooded expletives; It allowed us to evolve other advantages that build on that.
Not only we produce heat, we produce and let go of it in a very fine tuned, very delicately calibrated way so as to keep it in the best setting for our enzymes and proteins. A little hotter and we cook from inside out - literally! A little colder and we are not using our energy reserves in the most efficient way - we could take days to digest an hamburger in cold places if we were cold blooded.
Back to the core of the question. There are a few things that can cause a person to's body temperature to drop. From the wiki I already linked:
Hypothermia has two main types of causes. It classically occurs from exposure to extreme cold. It may also occur from any condition that decreases heat production or increases heat loss. Commonly this includes alcohol intoxication but may also include low blood sugar, anorexia, and advanced age.
People with hypothermia still produce some heat, they just don't produce enough of it. Hypothermia does lead to another, very common condition in which people do not produce any body heat: death.