The Ca and ADP ribose (ADPR)-activated cation channel TRPM2 is the closest homolog of the cold sensor TRPM8 but serves as a deep-brain warmth sensor. To unravel the molecular mechanism of heat sensing by the TRPM2 protein, we study here temperature dependence of TRPM2 currents in cell-free membrane patches across ranges of agonist concentrations. We find that channel gating remains strictly agonist-dependent even at 40°C: heating alone or in combination with just Ca, just ADPR, Ca + cyclic ADPR, or HO pretreatment only marginally activates TRPM2. For fully liganded TRPM2, pore opening is intrinsically endothermic, due to 10-fold larger activation enthalpy for opening (200 kJ/mol) than for closure (~20 kJ/mol). However, the temperature threshold is too high (>40°C) for unliganded but too low (<15°C) for fully liganded channels. Thus, warmth sensitivity around 37°C is restricted to narrow ranges of agonist concentrations. For ADPR, that range matches, but for Ca, it exceeds bulk cytosolic values. The supraphysiological [Ca] needed for TRPM2 warmth sensitivity is provided by Ca entering through the channel's pore. That positive feedback provides further strong amplification to the TRPM2 temperature response (Q ~ 1,000), enabling the TRPM2 protein to autonomously respond to tiny temperature fluctuations around 37°C. These functional data together with published structures suggest a molecular mechanism for opposite temperature dependences of two closely related channel proteins.