The Generalized Integral Transform Technique (GITT) is advanced to deal with conduction heat transfer in anisotropic heterogeneous media. A formal solution for exact integral transformation of conduction in anisotropic media is extended to account for heterogeneities expressed as space variable equation coefficients. The proposed eigenfunction expansion is based on biorthogonal eigenvalue problems which results in an exact integral transformation for linear problems and in a coupled transformed system for nonlinear situations. The associated eigenvalue problems are also handled through the GITT by considering simpler auxiliary biorthogonal eigenvalue problems of known analytical solution, leading to transformed algebraic eigenvalue problems. Finally, a test case is considered, typical of thermal metamaterials modeling, based on combining isotropic and anisotropic materials towards a final structure that implements a specific thermal function in a defined region of interest. A brief convergence analysis of the eigenfunctions expansions is provided.