A rigorous theoretical treatment of stimulated thermal scattering for an isotropic medium is presented. A phenomenological damping term is introduced into the electromagnetic field equation and then this equation is simultaneously solved with the equations describing the medium. As the set of equations is nonlinear, the solutions have been obtained under suitable approximations. The implications of these approximations, as far as performing the experiments is concerned, are thoroughly discussed. The nonlinear nature of the problem is responsible for saturation, and the optical-loss term gives rise to depletion. Both these phenomena have been included in our treatment. Whereas the 'saturation' is always present in a real system, 'depletion' can be an important effect in a medium involving a large linear optical absorption coefficient α. The experimental relevance of both these effects is discussed. Furthermore, we have derived the threshold condition on the laser field intensity. In the appropriate limits, our results reduce to those already reported in the literature.