83 sounds low, but then you are really short. I'm not sure what to say. I'm 5'8-3/4 and float around 115 tom 120 pounds. I've had a doctor tell me that I was not gaining weight fast enough, but then I've been like this for years and I haven't noticed anything bad happening. How long have you had this weight to height ratio? Have you experienced any of the symptoms of emaciation?
On one hand, it's better safe than sorry, so I woud encourage you to continue seriously looking into this. On the other hand, I see a lot of fat people, and my doctor was fat. I feel fine and have always been this way, but I am so abnormal in this society that I think it is essentially assumed that I'm unhealthy. It might be the same way with you.