I know from reading here you can have a functional B12 deficiency and have normal B12 levels. I have MTHFR, high cortisol, hypothyrodism, and 24/7 shortness of breath. I've been treating the high cortisol for months now and is better. My glucose levels are finally in the normal range and symptoms are lessening. The shortness of breath has been terrible. Thyroid labs are normal. Recently, I developed a terrible cough and thought it was probably my yearly bought with bronchitis. The difference was, I started getting rigors nightly and even ran fever a couple of times. The rigors didn't feel like normal chills. I couldn't get warm no matter what I did. I have also experienced unexplained weight loss over the last few months to the tune of over 20 lbs. I know a B12 deficiency can cause weight loss. So, I started taking a little methyl b12 along with 400mcgs of methylfolate. I started with 250mcgs of methylb12 and worked up to 2000mcgs. With the first dose, I couldn't believe how much better I felt. My lungs opened up and it was totally wonderful. I stopped feeling cold as well. I also started taking magnesium and calcium as well. I felt great for about a week 1/2 and could breath normally again which I hadn't been able to do in a long time. Then, the shortness of breath returned and was even worse. Stopped the B12. Had labs done. My B12 level was greater than 2000. My iron levels were normal. Should I be concerned about my B12 levels being so high? Also, because my symptoms disappeared after starting Methyl B12, I believe I do have a deficiency. Did the shortness of breath return because I was taking too much? It appears if I was deficient, 2000mcgs isn't that high of a dose.