Ok, well I have had this "disorder" most of my life(since like 4). The symptoms are: I cant gain weight, and my BMI is somewhere in the 16s zone. I mean I can gain weight by drinking tons of whole milk and eating butter, but even then my body just wont go BMI over 20 (I get total lack of appetite, puking/diarrhea and stuff like that).

I am now in the army, and have of course lost a lot of weight, since I am sick all the time and the army food just cant keep up with my higher than normal metabolism. The doc has noticed this and now thinks I have developed diabetes, which I think is complete misdiagnosis, since I have had issues with weight all my life in reality ( can you have diabetes for years?).

Anyway what disorder do you think I may have? Hyperthyroid perhaps? Some parasite?

I have hard time believing that its just genes, cause I would think genes would only get you to ranges like BMI 18-20 tops.