From a thermodynamic relation that involves the generalized relaxation function, an approximated method that allows estimation of the thermal dependence of the imaginary part of the general complex susceptibility from the real part is discussed. Several conditions such as broad distribution of relaxation times, the applicability of the time-temperature superposition principle, and the temperature dependence of the relaxation strength are considered and analyzed, together with the expected thermal behavior of the relaxed and unrelaxed susceptibilities. The method is tested by means of dielectric measurements on the poly(vinyl acetate) polymer in the temperature range corresponding to the segmental dynamics and the secondary relaxation. An acceptable agreement respect with the maximum temperature of the epsilon "(T) and the width and shape of the loss peak is obtained between the experimental and estimated imaginary part. This comparison gives validity to the method when the imaginary part is experimentally not well resolved due to the signal-resolution ratio or to the presence of other contributions that mask the thermal behavior of the imaginary part.