Well, it helps you sweat so it helps your body getting rid of toxins but I don't think there is much truth in changing metabolism.
logically, the heat will have your body work a bit harder to get the blood and oxygen around. The heat will make your blood a bit thicker and your arteries wider, your heart will work a bit harder thanks to that. I guess you could say that you burn a bit more calories thanks to that. However it will be a minimal change as it's only your heart that is working harder. the other muscles will be affected minimally. If they were Sauna's would be a bad idea for anyone with a chronic disease and a no-go-area for heart patients. (I know there are even health benefits for heart patients in a sauna)
The fact that you loose weight in a sauna is thanks to the fluids you loose in there. people can loose up to 3 or 4 pound of water due to sweating in a sauna. (I'm not saying that's healthy!)