Use and calibration of the internal temperature indicator (PDF!), an application note from Microchip:
Many PIC16 family devices include an internal temperature indicator. These devices include the PIC16F72X device family, PIC16F1XXX device family, and the PIC12F1XXX device family. The temperature indicator is internally connected to the input multiplexer of the ADC (Figure 1). Refer to the specific device data sheet for more details.
These devices incorporate an internal circuit which produces a variable output voltage with temperature using internal transistor junction threshold voltages. The indicator can be used to measure the device temperature between -40°C and +85°C. The circuit must be calibrated by the user to provide accurate results, as the equation coefficients will vary between devices.