In this paper, we consider Z r −graded modules on the Cl(X) −graded Cox ring C[x1, . . . , xr] of a smooth complete toric variety X. Using the theory of Klyachko filtrations in the reflexive case, we construct a collection of lattice polytopes codifying the multigraded Hilbert function of the module. We apply this approach to reflexive Z s+r+2graded modules over non-standard bigraded polynomial rings C[x0, . . . , xs, y0, . . . , yr]. In this case, we give sharp bounds for the multigraded regularity index of their multigraded Hilbert function, and a method to compute their Hilbert polynomial.