Answer to Question #7392 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 am going in for radioactive iodine treatment in just over two weeks. I will receive a 3700 MBq 131I dose. All the literature I've read only talks about how far away I should stay from humans, nothing about animals. I do not have children, but have a couple of cats that I am very concerned about exposing to radiation. What is the danger to them? If I'm supposed to keep my distance from children, shouldn't I also have to keep a distance from the cats as they are so much smaller than even the smallest child?

I've also found conflicting information regarding the amount of time that the bulk of the radioiodine will be excreted. I've read as little as 28 hours and as long as seven days. How long does it take for the bulk of it to be excreted?


The excretion time varies for every person, but the majority of an 131I dose is removed in the first eight hours. After that, the remainder, which is a small fraction of the administered dose, decays with approximately a seven-day half-life for patients who have high-dose therapy.1 So whatever is left in your thyroid after the first day will decay by half every week. The radiation that you expose your cats and others to is actually lower than the patients who receive doses less than 1221 MBq since you probably have little thyroid tissue left. You would have a lower uptake fraction, so there is less radioactive iodine in your thyroid, therefore less radiation exposure.

If you want to keep the radiation dose low to your cats, you shouldn't let them sleep next to your head or on your chest, but I think sitting on your lap is fine. What is more of a potential exposure to your cats is if you pet your cats and then they lick their coats, as most cats do. The iodine will come out of your pores, especially from your hands, for a few weeks. So you can either wear gloves or just wash your hands before petting your cats; I would suggest doing that for the first few weeks. Since cats probably have very small thyroids, you will want to avoid having them lick up your iodine off their fur when they groom themselves. Hand washing is very effective as long as you do it consistently. Even if your hands don't feel sweaty, there is always some sweat coming out of the pores.

Also, the iodine will come out onto your hair since your head sweats too. Keep the cats off your pillow since most people shed hair when they sleep. And don't let the cats lie where you lay your head either.

Follow these recommendations and your kitties will be just fine.

Marcia Hartman, MS

1 Half-life information from International Commission on Radiological Protection Report Number 53, Radiation Dose to Patients from Radiopharmaceuticals. March 1987.

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.
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