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We investigate the carrier density and temperature dependence of the Seebeck coefficient of via density-functional calculations and Boltzmann transport theory. The pertinent band structure has light bands at the band gap and heavy degenerate bands with band minima near energies corresponding to the experimentally determined optimum carrier density. Heavy bands increase the energy dependence of the density of states, which increases the magnitude of the Seebeck coefficient in an itinerant conduction regime, while the light bands provide a conduction channel that works against carrier localization promoted by La vacancies. The net result is thermoelectric performance greater than current -type materials above 1000 K.