A thermometer includes a processing unit configured to receive a plurality of timed apart temperature readings from first and second sensors and a processor calculates heat flux value, Q, and obtains values of the heat flux vs. temperature (Q vs. Ts.) as the temperature approaches a steady state. The processor empirically predicts the steady state temperature of the sensor Ts, using the peak value of the values of Q vs. Ts.The processor may also empirically calculate a bias value as a function of the peak value of Q vs. Ts. The bias value represents the difference between the temperature reading (Ts) at steady state and core temperature of the subject and is added to the steady state temperature to arrive at core. The thermometer probe may include one or more additional sensors for obtaining physiological readings from the subject other than temperature and the processing unit is configured to use empirical formulas to calculate the bias value using the physiological readings which may include blood perfusion, pulse rate and the bio-impedance of the subject.