Well, when I was found B12 deficient, neither the neurologist or my PCP seemed to care WHY . I think my PCP's response when I asked her was "some people are just low."

There are so many potential causes for B12 deficiency. I think it is important to find out, because there may be an underlying condition that needs treating, such as celiac disease or h. plyori infection.

The most common cause of B12 deficiency is pernicious anemia (another autoimmune disease where one lacks the intrinsic factor needed to process B12), and the antibodies they can test for that are below, but they say up to 25% with pernicious anemia won't show these antibodies.
intrinsic factor antibody
parietal cell antibody

Celiac Disease is another cause, and the full panel of tests include:
antigliadin IgA, IgG
anti-tTG
anti-endomysial
anti-reticulin
total IgA

H. Pylori infection is associated with B12 deficiency.

Gastric bypass.

Long term use of acid blockers.

There are many more.

If you check out the B12 deficiency page in The Gluten File you will find a couple good articles available online which cover the wide range of possibilities.

I hope you start to feel better soon. Many people do just fine with oral supplementation of B12, so you may want to talk to your doctor about switching to oral B12 for maintenance...many are unaware that oral dosing has been found effective. There is more on that in The Gluten File, too.

Cara