We investigate how spatial variations in tidal heating affect Io’s isostatic topography at long wavelengths. The difference between the hydrostatic shape implied by Io’s gravity field and its observed global shape is less than the latter’s 0.3 km uncertainty. Assuming Airy isostasy, degree-2 topography <300 m amplitude is only possible if surface heat flux varies spatially by <17% of the mean value. This is consistent with Io’s volcano distribution and is possible if tidal heat is generated within a convecting layer underneath the lithosphere. However, that layer would require a viscosity <10¹⁰ Pa s. A magma ocean would have low enough viscosity but would not generate enough tidal heat internally. Conversely, assuming Pratt isostasy, we find ~150 m degree-2 topography is easily achievable. If a magma ocean was present, Airy isostasy would dominate; we therefore conclude that Io is unlikely to possess a magma ocean.