Measurements of the electron temperature using standard diagnostics, like electron cyclotron emission or soft x-ray diagnostics, tend to be inadequate due to the natural limitations of these methods at low-toroidal magnetic field ( B 0 0.5 T) and plasma density [ n e ~(1-5)×10 18 m -3 ] in the typical discharges of the ISTTOK tokamak. On the other hand, the presence of a noticeable population of the suprathermal (runaway) electrons often causes a significant discrepancy between the values of the electron temperature yielded by different diagnostics. A numerical code was developed for the determination of the electron temperature from the measured macroscopic plasma parameters in discharges with and without runaway electrons. Simultaneous reconstruction of the time variation of the electron temperature and the runaway electron characteristics was carried out self-consistently using plasma power-energy balance calculations with the inclusion of the runaway process. Runaway electron instability caused the short-time relaxations of the plasma parameters detected by the ISTTOK data acquisition system. Analysis of the plasma parameters evolution during relaxations was used for the verification of the reconstruction procedure. The developed algorithm of the experimental data analysis was investigated in relation to its possible use for modeling of the runaway generation during a thermal quench in large-scale tokamak disruptions