Systems and methods of determining a core temperature of an internal organ in a body are provided. A thermal mass of known thermal resistance can be located between a body-contacting substrate and an external substrate. The body-contacting portion of the substrate can be placed over the organ whose temperature is to be measured. Temperature sensors can measure a temperature difference across the thermal mass and an absolute temperature at the external portion of the substrate. An electronic processor can receive the temperature measurements and can compute the core temperature of the organ based on the temperature measurements and the thermal resistances of the thermal mass and the tissue surrounding the organ.
