A theoretical investigation of the effective conductivity of a composite material containing carbon nanotubes (CNTs) as inclusions was carried out in the GHz and THz frequency ranges. First, the electromagnetic response of an isolated pristine or doped CNT of finite length was obtained using an integral-equation technique. Then the Waterman-Truell homogenization formalism was implemented to estimate the effective conductivity. A physical interpretation of experimentally observed terahertz peak of the effective conducitivity emerged from our calculations. The influences of substitutional doping of the CNT inclusions and the stretching of the host material on the effective conductivity were deduced.