Pions at Finite Temperature from QCD Sum Rules
  The temperature corrections to the current algebra Gell-Mann, Oakes, and Renner (GMOR) relation in $SU(2)\otimes SU(2)$ are investigated in the framework of QCD sum rules. There are no corrections at leading order in the quark masses. At the next to leading order we find corrections of the form $m_{q}^2\; T^{2}$, which are small except near the critical temperature. As a by-product we obtain the temperature behaviour of the pion mass, which is essentially constant, except near the critical temperature where it increases with $T$. 
C.A.Dominguez, M.S.Fetea, and M. Loewe