Possible, yes. I'm 5'7 and I've gotten down before to 100lbs, with my body still functioning pretty normal (ie.still having periods), and still having boobs and not having countable ribs. But I was always freezing, and sitting on hard surfaces, like a toilet, hurt. But I was probably right at the tipping point, or would have gone over the (kinda) healthy line if I had remained that weight longer than the 3-4 months I did. Some people do have smaller bone frames. Though, as we all know a fph, they come in only a small range of possible sizes. (And this applies in both directions.)