The poor stability of organometallic halide perovskites, especially CH3NH3PbI3 (MAPbI3), in high-temperature environments is one of the challenges retarding its wider applicability. A better understanding of how MAPbI3 perovskite degrades thermally will allow us to develop strategies to improve its "intrinsic"stability. In this study, we employed first-principles density functional theory to obtain detailed thermal degradation energy landscapes for MAPbI3. We focused on studying two reaction pathways: P1, for proton abstraction from methylammonium (MA) to iodine, and P2, for substitution (SN2) of an iodide anion (I-, as nucleophile) at the carbon atom of MA to yield iodomethane (CH3I) and ammonia (NH3). Our calculations provided kinetic data, allowing us to assess the "intrinsic"(in)stability of MAPbI3 under heat stress and to make sense of the seemingly contradictory experimental results. The I- species is more reactive for reaction P1 but not for P2. The energy of the reaction along pathway P1 increases monotonically with respect to proton abstraction. The reaction along pathway P2 is a two-step process, with a CH3-I-Pb-X intermediate formed in the crystal in conjunction with the release of NH3 gas. Our results suggest that the method of preparation (e.g., iodine-deficient conditions) and the morphology (e.g., improved crystallinity) of MAPbI3 could both influence its thermal stability.