ailments there is serious loss of blood protein through the intestines. In these conditions the ⁵¹Cr level in the intestinal excretions is high, and this alerts the doctor to apply remedial measures.

Cobalt-60

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.

Iodine-131

Like chromium-51, iodine is a versatile tracer element. It is used to determine blood volume, cardiac output, plasma volume, liver activity, fat metabolism, thyroid cancer metastases, brain tumors, and the size, shape, and activity of the thyroid gland.

A linear photoscanner produced these pictures of (A) a normal thyroid, (B) an enlarged thyroid, and (C) a cancerous thyroid.

Because of its unique connection with the thyroid gland, iodine-131 is most valuable in measurements connected with that organ. Thyroxin, an iodine compound, is manufactured in the thyroid gland, and transferred by the blood stream to the body tissues. The thyroxin helps to govern the oxygen consumption of the body and therefore helps control its metabolism. Proper production of thyroxin is essential to the proper utilization of nutrients. Lowered metabolism means increased body weight. Lowered thyroid activity may mean expansion of the gland, causing one form of goiter.

Iodine-131 behaves in the body just as the natural non-radioactive isotope, iodine-127, does, but the radioactivity permits observation from outside the body with some form of radiation counter. Iodine can exist in the body in many different chemical compounds, and the counter can tell where it is but not in what form. Hence chemical manipulation is necessary in applying this technique to different diagnostic procedures.

The thyroid gland, which is located at the base of the neck, is very efficient in trapping inorganic iodide from the blood stream, concentrating and storing the iodine-containing material and gradually releasing it to the blood stream in the form of protein-bound iodine (PBI).

One of the common diagnostic procedures for determining thyroid function, therefore, is to measure the percentage of an administered dose of ¹³¹I that is taken up by the gland. Usually the patient is given a very small dose of radioactive sodium iodide solution to drink, and two hours later the amount of iodine in the gland is determined by measuring the radiation coming from the neck area. In hyperthyroidism, or high thyroid gland activity, the gland removes iodide ions from the blood stream more rapidly than normal.