The stability of drift-tearing modes in the collisional and semi-collisional regimes is studied analytically, when thermal effects are included. We use a fluid description of a cylindrical plasma and assume a variable plasma resistivity, depending on the local power input. The linear growth rates for the thermally modified tearing modes (Δ' > 0) and for the thermal instabilities (Δ' < 0) are derived. The saturation mechanisms for these instabilities are also discussed.