Answer to Question #7138 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:


I had a total thyroidectomy due to papillary carcinoma. A month later, I received 3700 MBq of 131I. When I was discharged from the hospital three days later, the nuclear medicine technologist measured the remaining amount of radiation in my body and he got 7.4 MBq. It's now another three days later. When is the safest time for me to resume normal interactions with my children? My youngest son can't sleep well because I always hug him until he sleeps. I hope you can help me.


Iodine-131 (131I) therapy used for treating thyroid cancer involves two issues that control exposure to family members or the public in general. The first issue is the possibility of passing on radioactive contamination to others and the second issue is exposure to radiation from the residual 131I that is retained in the body.

Iodine-131 is orally administered in either a liquid form or as a capsule(s). Chemically, the 131I is in the form of sodium iodide, which is a salt similar to normal table salt, sodium chloride. Thyroid tissue is the only tissue in the body that will naturally concentrate and hold the iodine. The rest of the tissues in the body see the iodine only as a salt and it is rapidly cleared from the body, primarily via the urinary system.

Someone like you, who has had a thyroidectomy, can only hold on to a small fraction of the 131I, usually less than 1 percent of the administered dose. Usually, in people with good kidney function, about half of the administered activity will be excreted within the first 8 to 10 hours. After two days, over 95 percent of the radioactivity is already excreted from the body. In your case, after three days, 99.8 percent of the radioactivity was excreted. During this initial excretion phase, patients are commonly either isolated in the hospital or instructed to follow precautions to prevent activities that might allow contaminated body fluids to be exchanged with family or friends. After this initial excretion phase, the ability to pass contamination on to others is greatly diminished, because the 131I becomes fixed in the residual thyroidal tissues and is no longer rapidly excreted.

The remaining 131I in the thyroid gland after this excretion phase is only a small exposure source after two to three days and doesn't pose a significant source of radiation to your child. After this point, even sleeping with your child for eight hours every night would cause an additional dose of approximately 10 microsievert/day, which is comparable to the average natural background dose per day. There is no evidence that this additional dose has any effect, even for children. However, to be conservative it is usually recommended that contact with children be limited for the first week after treatment.

The residual 131I in the thyroid will continue to decrease, losing half of its activity every five to seven days. So, by the time you read this (10 days post administration), the potential dose will be even smaller. So, to make a long story short, it is now safe for you to resume normal interaction with your children. I hope you and your youngest son can now get some rest!

Mike Bohan, RSO

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
Answer posted on 22 January 2008. 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.