The reversible mixed state of the Tl2Ba2CaCu2O8 (Tl-2212) high Tc superconductor with Tc of 107K has been investigated by direct measurement of the AC differential susceptibility ρ'H which reduces to the equilibrium dM/dH when, as it is actually the present case in the reversible regime, the flux flow viscous contribution to ρ'H could be neglected. The following results have been obtained: a strong (field enhanced) contribution from diamagnetic fluctuation above Tc, while below Tc one observes deviation from the Abrikosov mean field theory of the mixed state which is presumably generated by thermal fluctuation enhanced effect on the vortex density and distortions in the layered structure. This behaviour appears to be common to all the high Tc superconductors as we have shown in an earlier work for the YBa2Cu3O7−x, Tl-2201, Tl-2223 and Bi-2212 compounds. Moreover, these deviations prevent any reliable determination of the Hc2 critical field line by the standard method of linear extrapolation of M(H,T) curves by the step anomaly on ρ'H expected to occur at Tc(H) if the mean field theory was to be followed in the mixed state.