It depends what you mean by "isoterm altitude".
"Surface temperature" is at 2m above ground. Respectivelly 2m above model terrain.
Higher temperature levels are placed in certain "isobaric levels". If you want to recalculate the isobaric level to altitude, you can switch to "Geopotential height" overlay which shows you the altitude (AMSL) of the specified isobaric surface.
If you are asking for the altitude of zero-degree isotherm, you are lucky man/woman, because we are planning to add it as a new overlay soon - maybe next week.