Renormalization and radiative corrections in QED are studied at finite temperature and density. We use the Fermi-Dirac and Bose-Einestein distribution functions in a real-time formalism to generalize the calculations of the electron mass, wavefunction and charge renormalization constants, at high temperatures and chemical potentials. Further, the radiative corrections to the decay rate of a neutral scalar particle H → e⁺e⁻ and the leptonic decay of Z boson are re-examined at high temperatures and chemical potentials. Renormalization constants of QED are calculated for μ > T and T> μ in the limit T→0 and μ < m. We also study the effects of the statistically corrected physical mass of electron on its anomalous magnetic moment and on the beta decay processes affecting the parameters of primordial nucleosynthesis. It is shown that the helium abundance parameter, energy density of the universe and the leptonic decay rates become implicit functions of temperature and density. Density effects are not important from the point of view of the early universe. However, these results are expected to be applicable to superdense stars.