The refractive indices and the cutoff conditions for electromagnetic waves in plasmas are investigated for cold, hot and relativistic plasma models. Significant relativistic mod ifications of refractive indices and locations of cutoffs are found in regimes relevant for reflectometry in large Tokamaks. For X-mode it is demonstrated that these effects may shift the location of the reflecting layer by a significant fraction of the minor radius and that the cold model may lead to considerable under estimations of the density profile. Relativistic effects predicted for O-mode reflectometry are smaller than for X-mode, but not negligible. An algorithm for reconstruction of density profiles which allows a relativistic plasma model to be used is presented.