Answer to Question #7138 Submitted to "Ask the Experts"

I had a total thyroidectomy due to papillary carcinoma. A month later, I received 3,700 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% 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% of the radioactivity is already excreted from the body. In your case, after three days, 99.8% 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