Answer to Question #8500 Submitted to "Ask the Experts"

Category: Nuclear Medicine Patient Issues — Therapeutic Nuclear Medicine

The following question was answered by an expert in the appropriate field:


My partner has had radioiodine (iodine-131) treatment for hyperthyroidism. I was reading through the questions and answers on your Web site and am seeking clarification.

The answer to the question "What precautions need to be taken when a family member or I have radioiodine therapy that uses iodine-131 and go home?" seems ambiguous.

I'm quoting from the answer: "The total radiation dose to anyone else, even with close, continuous contact, will not cause harmful effects. There is no need for concern about effects on your family, pets, etc., but it is prudent to avoid close, prolonged contact for the first week."

They don't seem to be compatible statements. Why be prudent if there are no ill effects? And further, how do I reconcile the recommendation to not sleep in the same bed for two nights with either of those statements?

I seem to have had trouble finding clear guidelines on this. Can you please clarify at all?


The regulations of the Nuclear Regulatory Commission (NRC)1 and most states allow hospitals to release patients who have received treatments, such as your partner, if members of the general public (which includes family members) are not likely to receive an effective dose equivalent of radiation in excess of 5 millisieverts (a millisievert is a unit of effective dose equivalent). The NRC regulations also allow individuals who use radiation in their work to legally receive up to 50 millisieverts per year.

To put those numbers in perspective, according to the National Council on Radiation Protection and Measurements (NCRP)2, the average effective dose to a member of the U.S. population is a little over 3 millisieverts per year from "ubiquitous background," which includes radiation from outer space, the sun, and naturally occurring sources on the earth such as radioactive materials in the soil and rocks and radon gas. This number can vary by as much as a factor of two, depending upon where you live and the radon levels in your home.

Radiation doses at very high levels (in the thousands of millisieverts) can cause observable, short-term effects. In fact, that is the whole idea behind radioiodine treatment—to cause an effect in the thyroid. Statistical studies have also shown that radiation doses in the hundreds of millisieverts can also cause long-term effects such as cancers and leukemia; however, it has yet to be determined the lowest radiation dose required (i.e., the "threshold" dose) to cause such long-term effects. It is like asking the question, "How many cigarettes does one have to smoke to cause lung cancer?" Studies have shown that smoking increases one's risk of lung cancer, although the definitive number of cigarettes that actually causes cancer in a specific individual cannot be determined. According to a position statement from the Health Physics Society3 (HPS) the risks of health effects from dose equivalents below 50 to 100 millisieverts are either too small to be observed or are nonexistent.

In radiation protection, we use a concept called "As Low As Reasonably Achievable" or "ALARA." This concept acknowledges that while, as the HPS states, the risks of health effects may be too small to observe at very low doses, it makes sense to minimize even low doses if it is reasonable to do so. Thus, in the information you read from the Web site, they are basically saying that the risk of harmful effects to family members is infinitesimally small, but in keeping with the ALARA concept, it is prudent (or reasonable) to limit family members' dose even further by avoiding close contact and sleeping alone for a few days following treatment. As your health care personnel most likely told you, the amount of radioactivity in your partner's body will decrease with time and you can discontinue precautions after a few days.

Finally, all hospitals where radioiodine therapy treatments are performed have an individual designated as the radiation safety officer. You should feel free to ask questions of either that individual or the physician who provided treatment.

Mack L. Richard, MS, CHP - Radiation Safety Officer
Indiana University Medical Center

1Title 10 - Energy, Code of Federal Regulations

2 NCRP Report No. 160, "Ionizing Radiation Exposure of the Population of the United States"

3Position Statement of the Health Physics Society (PS010-1), "Radiation Risk in Perspective"

Answer posted on 15 September 2009. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.