In the past, few studies have attempted to determine some of the internal properties of Coronal Mass Ejections (CMEs), which were limited to a certain position or a certain time. For understanding the evolution of internal thermodynamic state of CMEs during their heliospheric propagation, we improve the self-similar flux rope internal state (FRIS) model, which is constrained by measured propagation and expansion speed profiles of a CME. We implement the FRIS model to a CME erupted on 2008 December 12 and probe the internal state of the CME. We find that the polytropic index of the CME plasma decreased continuously from 1.8 to 1.35 as the CME propagated out from 6 Rs to 14 Rs, implying that the CME released heat before it reached adiabatic state and then absorbed heat. We also estimate the entropy changing and heating rate of the CME. Our results show that the thermal force inside the CME is the internal driver of CME expansion while Lorentz force prevented the CME from expanding. It is noted that centrifugal force originated due to poloidal motion of the plasma inside the flux rope decreased with fastest rate and Lorentz force decreased slightly faster than thermal pressure force as the CME moved away from the Sun. We further extrapolate the internal thermodynamic properties of the CME up to 1 AU and compare with in situ observations. The limitations of the model and approximations made in the study would also be discussed.