Pernicious anemia is caused by a deficiency of vitamin B12, which is needed for normal production of red blood cells. It is often hereditary. Risk factors include a history of autoimmune endocrine disorders, a family history of pernicious anemia, and Scandinavian or Northern European descent.
The meat and dairy products we eat are our primary sources of vitamin B12. However, except in strict vegetarians, pernicious anemia isn't simply caused by not eating enough of these foods. Usually, it is because of a failure in the complex process the digestive tract must go through to absorb vitamin B12.
In order for vitamin B12 to be absorbed by the small intestine, the cells that line a part of the stomach must produce a substance called intrinsic factor (IF). This substance attaches itself to vitamin B12, and both are absorbed in combination into the lowest portion of the small bowel (ileum), just before the small bowel enters the colon. If the ileum is damaged or removed in the course of surgery, the intrinsic factor/vitamin B12 combination will not be absorbed. People with conditions like Crohn's disease, who often have surgery to remove part of their ileum (the part of the small intestine where vitamin B12 is absorbed), should be screened for vitamin B12 deficiency and treated if needed.
Lack of intrinsic factor may also be congenital (present at birth). This form of pernicious anemia (called juvenile or congenital) is usually seen before a child is three years old. It is believed that only one parent needs to carry the gene for this disorder to pass it along to a child.
Less common causes of decreased B12 absorption include chronic pancreatitis, malabsorption syndromes, certain medications, and, very rarely, increased metabolism of B12 through longstanding hyperthyroidism. A very common cause of B12 deficiency in the elderly is inadequate absorption of dietary B12.
Pernicious anemia is often also seen in combination with some autoimmune endocrine (gland) diseases such as type 1 diabetes, hypoparathyroidism, Addison's disease, and testicular dysfunction.