The normal blood level of vitamin B12 in adults is 200 to 900 picograms per milliliter, according to MedlinePlus. Values below 200 picograms per milliliter indicate vitamin B12 deficiency in most adults, although elderly people with levels from 200 to 500 picograms per milliliter may experience deficiency symptoms.
Malabsorption diseases, such as Crohn's disease and celiac disease, can cause B12 deficiency, reports MedlinePlus. Hyperthyroidism, pregnancy and the absence of a protein that the intestines need to absorb vitamin B12 are other causes. Lack of a sufficient amount of B12 in the diet may also cause deficiency, although this usually only occurs in people following strict vegetarian diets.