As an instrumentation engineer, I have designed temperature control systems capable of measuring and controlling temperature with a precision of 0.001K over a wide temperature range. I have always wondered how the human body maintains its temperature within a fraction of a degree; specifically, where and how is the temperature measurement done?
$\begingroup$ en.wikipedia.org/wiki/Thermoregulation#In_humans $\endgroup$– MemmingOct 20, 2014 at 14:32
$\begingroup$ en.wikipedia.org/wiki/Thermoreceptor $\endgroup$– inf3rnoOct 20, 2014 at 18:22
$\begingroup$ As @inf3rno said there are thermoreceptors (TRPV ion channels) that are responsive to temperature. They are involved in temperature sensation of the environment as well as the body itself. I am not fully sure if they are the sole temperature sensors of the body. Have a look at these articles: ,  and  $\endgroup$– WYSIWYGOct 21, 2014 at 5:09
Human thermo detection comes from a protein, expressed in high levels in the nervous system.
As per comments, there are a group of transmembrane proteins called transient receptor potential cation channel subfamily V (TRPV) ion channels that can detect temperature changes. Although the exact mechanism of the protein remains unclear, a study published in Nature 2010 showed that a protein called TRPV1 opens and closes to allow increased or decreased sensitivity to temperature change.
There is no specific organ that is responsible for temperature detection, however TRPV1 is expressed in high levels in the nervous system.