If you go to the right doctor, he can take a video camera and look at your scalp under magnification to see if your hair is miniaturizing in certain areas, to see if you have mpb. I'll defer to someone with more knowledge, but normally a scalp biopsy would not be done just because someone's hair was thinning. There is no blood test that will indicate whether you will keep your hair or lose it. 5'7, 110 lbs is very thin. It is highly unlikely that there is any relationship between your weight and your hair, but in the interest of overall health and well being, why don't you try having one additional meal each day and a multi-vitamin.