It doesn't produce an electron, it just donates its energy to one of the electrons bound to it. This energy is then used to free the electron from its bounds and any extra energy is used to give the electron some speed. Effectively the same energy is consumed when emitting the photon or releasing the electron. If the thorium is sufficiently ionized (the more electrons you strip from an atom, the harder it gets to strip the next one), the binding energy of the electrons is so high that the energy from the nuclear excited state is not enough to free the electron and the process is completely blocked from happening.