Using first principles density functional theory, we report for CrSi₂, a linear relationship between thermodynamic charge state transition levels of defects and maxima of thermopower T_m, thus proposing a unique way of tuning thermoelectric properties. We show for doped CrSi₂ that the peak of thermopower occurs at the temperature which corresponds to the position of the defect transition level. Therefore, by modifying the defect transition level, a thermoelectric material with a given operational temperature can be designed.