Iodine-131 Medical Use

Radioactive iodine (I-131) can be used in medical therapies as a treatment and a diagnostic tool. It is most commonly used in the treatment of hyperthyroidism due to Graves’ disease or a nodule in the thyroid gland.

Administration

I-131 can be therapeutically administered as a single capsule that is to be taken orally. The radioactive iodine emits both beta and gamma radiation. The beta radiation is the active agent that kills the overactive thyroid cells and travels small distances, not usually exiting from the patient’s body. The gamma radiation can travel several meters away from the patient and may affect other individuals in the vicinity.

At the therapeutic dose of iodine-131 most commonly used in therapeutic regimens, the gamma radiation has usually reduced to trace amount three days after administration.

Pharmacology

I-131 is a radioactive iodine salt that alters the mechanism of iodine absorption in the thyroid gland. Its use is particularly useful in the destruction of cells in the thyroid gland that are overactive. It can also be used in diagnostic imaging techniques used for pheochromocytoma and neuroblastoma.

It is the beta radiation from I-131 that provides the therapeutic benefit of the treatment. The majority (90%) of the effect is carried out by this radiation, although the gamma radiation also plays a role.

Adverse Effects

Treatment with radioactive iodine in therapeutic doses is well tolerated and is considered to be a safe and effective treatment for hyperthyroidism. The most common adverse effect is hypothyroidism, due to the excessive destruction of thyroid cells that lead to under activity of the gland.

Some patients with severe hyperthyroidism may experience worsening of symptoms immediately following the treatment. This occurs as a result of additional thyroid hormone that is released into the blood stream as the thyroid cells undergo necrosis. Symptomatic relief may be offered with short-term use of anti-thyroid medication for several months.

Exposure to radioactive iodine also has the potential to increase the risk of cancer of the thyroid gland or other areas of the body in patients.

Post-Treatment Isolation and Monitoring

Due to the exposure to I-131 in patients receiving therapeutic doses, there is a need for some level of isolation following therapy to minimize the risk to surrounding individuals.

The majority of the radioactive iodine is eliminated from the body through natural decay in the first 3-5 days following administration, with total elimination several weeks later.

The following methods help to reduce the effect of the radiation:

• Clean toilets and sinks regularly
• Wash bed sheets often
• Wear slippers and socks to increase physical isolation from others
• Use decontaminant specifically manufactured for radioactive iodine removal (avoid chlorine and bleach solutions)

It is recommended that patient should refrain from having sexual intercourse for one month following the administration of therapy. Female patients should use appropriate methods of contraception for six months to prevent possible risks of radiation to a developing fetus. These recommendations are conservative to eliminate the possible risk.

References

• http://www.radiologyinfo.org/en/info.cfm?pg=radioiodine
• http://www.insideradiology.com.au/pages/view.php?T_id=44#.VhLPqs7RifQ
• http://www.laradioactivite.com/en/site/pages/Iodine_131.htm
• http://www.newcastlenuclearmedicine.com.au/site/index.cfm?display=105594