You aren't supposed to heat thickened formula at all - this is the advice from 2 Drs, 3 ECHNs and also from the Wyeth careline. It defeats the purpose, the heat thickens the milk. It is supposed to be pretty similar to normal formula as they drink it & then thickened up in their tummy which makes it heavier in there & less likely to be thrown up. If it's thickened beforehand they are less likely to drink as much as they need, it's harder to drink so they will get frustrated & it can lead to them sucking in air as they struggle to get it through the teat.

I really really recommend that you don't heat bottles of thickened formula. Room temperature is not cold and over summer it can be quite warm....
And for that matter if you are heating bottles of any sort of formula in a microwave, it's recommended to heat the water first and then add the powder so none of the nutrients get zapped away.