In this study, the feasibility of using a thermoluminescent slab dosimeter (TLSD) in postal dosimetry audit for radiotherapy systems is examined. First, changes in material structure are evaluated by measuring changes in TLSD weight and temperature after heating and cooling. Then, a fading correction factor is derived by measuring the relationship between the elapsed time after irradiation and thermoluminescence (TL) intensity. Finally, the uncertainty in the absorbed dosimetry using the TLSD with the TL absorbed dose correction factor is evaluated. It is shown that the TLSD weight and heat flow change after heating. These changes are reversible upon cooling. Therefore, the material structure of the TLSD does not change after heating. The fading effect for the TLSD can be minimized by setting the elapsed time to more than 100 h. Furthermore, measurement accuracy can be improved by correcting the fading. The overall measurement uncertainty or expanded relative standard uncertainty for this measurement method is 1.80%. The TLSD has high material stability and measurement accuracy under a certain condition equivalent to that of conventional postal dosimetry audit conducted using a glass dosimeter. The TLSD can be used as a 2D measurement device for postal dosimetry audit.