Vitamin B₁₂ is a cobalt compound. Normally the few milligrams of B₁₂ in
the body are stored in the liver and released to the blood stream as
needed. In _pernicious anemia_, a potentially fatal but curable disease,
the B₁₂ content of the blood falls from the usual level of 300-900
micromicrograms per milliliter (ml) to 0 to 100 micromicrograms per ml.
The administration of massive doses of B₁₂ is the only known remedy for
this condition.
If the B₁₂ is labeled with radioactive cobalt, its passage into the
blood stream may be observed by several different methods. The simplest
is to give the B₁₂ by mouth, and after about 8 hours study the level of
cobalt radioactivity in the blood. Cobalt-60 has been used for several
years, but recently cobalt-58 has been found more satisfactory. It has a
half-life of 72 days while ⁶⁰Co has a 5.3-year half-life. This reduces
greatly the amount of radiation to the patient’s liver by the retained
radioactivity.