It's a really good question.
It can be elevated in people with MS, rheumatoid arthritis, gluten related neurologic disease, psoriasis, Sjogrens, SLE, thyroid disease, Down Syndrome, schizophrenia and more. And they say in 10-15% of 'normals' (although I have to question the definition for normal, because even those with biopsy proven celiac disease can...