We report on the threshold voltage shift and its origin in amorphous-silicon thin-film transistors with a nitride gate insulator. These transistors were subjected to two different degradation methods: (i) applying a strong gate bias at 450 K for a prolonged period of time (thermal bias annealing), and (ii) pulse light soaking at room temperature. To characterize metastable defects in these transistors, we propose and demonstrate the usefulness of spin-dependent photoconductivity (SDPC). For the light-soaked samples both a threshold voltage shift and an increase in defect density are observed, in contrast to the thermal-bias-annealing (TBA) treatments where threshold voltage shifts are observed, but no change in the defect density is detected with SDPC. The results are consistent with the main effect of TBA being the creation of states in the lower part of the band gap.