  In a previous paper (JHEP {\bf 05} (2014) 27), we calculated the three-loop
thermodynamic potential of QCD at finite temperature $T$ and quark chemical
potentials $\mu_q$ using the hard-thermal-loop perturbation theory (HTLpt)
reorganization of finite temperature and density QCD. The result allows us to
study the thermodynamics of QCD at finite temperature and isospin chemical
potential $\mu_I$. We calculate the pressure, energy density, and entropy
density, the trace anomaly, and the speed of sound at zero and nonzero $\mu_I$.
The second, fourth, and sixth-order isospin susceptibilities are calculated at
zero $\mu_I$. Our results can be directly compared to lattice QCD without
Taylor expansions around $\mu_q=0$ since QCD has no sign problem at finite
isospin chemical potential.
