A method and apparatus for both heating a patient's skin and for measuring the temperature using the same device, such as a thermistor. Thus, the thermistor generates controlled heat, and is not just used for sensing the temperature. In an oximetry sensor, the thermistor is located in the vicinity of the light emitter and photodetector to warm the optically-probed tissue region. As heat is dissipated, temperature changes are sensed as resistance changes according to Ohm's law. Active thermal regulation by varying the amount of thermistor current and power can safeguard against burning the tissue while maximizing perfusion.