  Equation of state and electric (isospin) charge fluctuations are studied for
matter composed of interacting pions. The pion matter is described by self
interacting scalar fields via a $\phi^4-\phi^6$ type Lagrangian. The mean-field
approximation is used, and interaction parameters are fixed by fitting lattice
QCD results on the isospin density as a function of the isospin chemical
potential at zero temperature. Two scenarios for fixing the model parameters --
with and without the first order phase transition -- are considered, both
yielding a satisfactory description of the lattice data. Thermodynamic
functions and isospin charge fluctuations are studied and systematically
compared for these two scenarios, yielding qualitative differences in the
behavior of isospin charge susceptibilities. These differences can be probed by
lattice simulations at temperatures $T \lesssim 100$ MeV.
