Speaking from experience, I think Bikram Yoga is really dangerous, especially if you are in any way metabolically supressed, and I’m speaking from experience. The summer that I started doing Bikram Yoga was the first summer that I noticed my body temperatures plummet to freakish lows (96-97) and my blood pressure drop to zombie status (low 90s/50s). I’m not prepared to say that the heat of Bikram Yoga has a specific effect on body temperature, but more that Bikram Yoga is extremely stressful on the body, which is extremely damaging — to metabolism and many other bodily processes. Exercising and stretching your muscles under unusually hot (and usually humid) conditions is not actually condusive to better health or fitness!
I wish I’d realized all this earlier and hadn’t wasted so much time (and $$) on Bikram, particularly because I think it took 3x the time spent practicing Bikram for my body to recover from the damage it did. The first few times you do Bikram you feel like you’re going to puke/faint/die, but if you endure the torture you get used to it and actually start to like the feeling. I remember after a month I felt like the Bikram classes weren’t “doing anything” anymore, so I started to walk the 2.5 miles to/from the studio instead of taking the bus (I was living in Manhattan at the time). This is all an indication that my body was experiencing extreme physical stress (hence the initial puking/fainting/dying), and in order to combat what my body had learned would be a constant state (because I continued to do Bikram despite all those negative feelings), it went into hyper stress-response mode (hence the initial feelings of maniacal energy and endurance) and ultimately entered a low-metabolic state (hence the low temp/BP and ultimately poor health symptoms).
And this is not to speak of the dangers to your muscle fibers of overstretching — which is really really easy to do when practicing yoga in unusually hot environments. Take care of your body, stay away from Bikram! :)