This depends on the capability of the thermographic camera used.
Standard uncooled cameras fitted with a microbolometer detector allow non-contact temperature measurement as low as -40°C. Below this temperature, they are still emitting in the infrared region, however due to Planck’s Law the infrared intensity emitted by objects is very low indeed. To measure below this limit is possible by using a VLWIR camera like the Flir SC7900VL. With a camera like this it should be possible to get calibrated measurement down as low as -60°C, possibly a little lower. However, by reducing the integration time and using techniques like FFT it certainly should be possible to see objects below this level, accurate measurement however would be unlikely.