  In this work we rederive the Lamb-Retherford energy shift for an atomic
electron in the presence of a thermal radiation. Using the Dalibard, Dupont-Roc
and Cohen-Tannoudji (DDC) formalism, where physical observables are expressed
as convolutions of suitable statistical functions, we construct the
electromagnetic field propagator of Thermo Field Dynamics in the Coulomb gauge
in order to investigate finite temperature effects on the atomic energy levels.
In the same context, we also analyze the problem of the ground state stability.
