Q: would using a sauna help me to lose weight.

A: No. While saunas can be great for your general well-being, they will not help you lose weight or lose fat. All the weight you lose in the sauna is due to sweat, so it's just water weight. Once you get out of the sauna and resume your regular routine, your body will rehydrate, get back the water it lost and your weight will be back to where it's started. Saunas are good for you and I do not mean to discourage them in any way. They are just not going to help you lose weight.

While on this topic, note that gimmicks like running out in the heat of the day, running in heavy clothing or running in rubber/plastic suits are all completely useless in trying to lose weight. Running itself will bring you some benefits when done in moderation, but making yourself sweat more while running has absolutely no benefit and can be very dangerous taken to the extreme (such as rubber/plastic suits). Sweat is not fat and sweating more does not burn any more fat. Remember to always stay hydrated while running, especially in hot weather.