It has recently been shown that an inhomogeneous, anisotropic Hall coefficient, arising from a charge-density-wave domain structure, explains the nonsaturating electrical magnetoresistance of potassium. It is shown here that the same mechanism also explains the observed behavior of the thermal magnetoresistance. The transverse thermal magnetoresistance of a domain structure increases with increasing field, having both a linear and quadratic component. The longitudinal thermal magnetoresistance of a domain structure initially increases linearly with increasing field. Its behavior in higher fields, however, depends on whether or not the domain distribution is symmetric about the field direction. If the distribution is symmetric, saturation occurs; otherwise, a residual increase is possible.