if you can post or email the blood tests, I would be glad to review them and help you
Dr Carol Osborne, DVM
For me this is a toughy. My first thought would be parasite, and in the back of my head I still want to say possible parasite, because not all parasites are excreted in fecal matter, which would explain a clean stool sample. When was the last time she was dewormed? Are you alternating the dewormer if your using paste(oral). Is she dewormed by paste or tube(nasal)? If you are using a paste wormer I would suggest doing a tube wormer just to see if that helps any, as paste wormers are not as effective for all parasites.
The high levels of calcium could be a symptom of a kidney issue, but from my years of experience I have not known a horse to lose weight with kidney issues. How does her urine look? Is it darker than normal, and does it have any distinct odor, any vaginal discharge? Has she experienced her first heat cycle yet? As clear fluid from the teets is pretty normal in mares that are in heat.
Have you had a dental exam done? A chipped or sore tooth can cause issues with her chewing. Have you changed her feed at all? Maybe an allergy to a certain cut of hay or type of grain/oat? Does she suck air(cribbing)? I know horses who have cribbed before and they too had weight issues. A little more background on her would be helpful in trying to give out options on things you can look into. How many hands is she so far, I know a belgium is a draft and are quite a bit larger than the average QH.
I would also ask to have her liver enzymes checked for elevation and to do a CBC if she hasn't had one done, and a white blood cell count as well, if her white blood cells are elevated, that is a sure sign that her body is trying to fight off some type of infection/disease.