What's the Radiation Exposure to Animals after Owner's Radioiodine Treatment?

I am suffering from hyperthyroid due to Graves' disease, and I'm being treated with radioactive iodine next week.

My doctor says that I will be able to go right home after my treatment, but that I shouldn't sleep with my husband for at least 5 days.  I don't have any children, but I do have my two male cats, aged 3 and 6 years.  I asked my doctor about my cat's exposure, but he didn't seem to know (or care).

Is there any problem with me being around my cats after I get treated? Can I make my cats (or husband) radioactive? If so, what precautions should I take so that doesn't happen.

My Response:
This is a common question that I get. Once you are treated with radioiodine and return home to your husband and cats, they are all going to be exposed to the gamma radiation emitted by you for a period of 2-3 weeks or so, depending on the dose of I-131 administered.

Although it highly unlikely that this radiation exposure would do harm to your family members, we want to minimize the amount of exposure to as low as reasonably possible. If this radiation exposure was certain to cause harm or damage, I would certainly be in deep trouble. Remember that I have been exposed to the radiation emitted from my hyperthyroid cats for the 32 years that I have treating them with radioiodine, and I still feel fine!

Radiation safety recommendations
To minimize the gamma radiation exposure to your cats, I would treat them exactly like I would treat any other member of the family (1-4). To that end, I recommend that you follow the American Thyroid Association's guidelines and take the following steps to help reduce the radiation exposure to your cats (and husband) after your treatment:

• Sleep in a separate bed for at least 1 week (2 weeks is better), especially if your cats sleep within 3-6 feet of your body.
• Maintain a distance of a least a 3-6 feet from your cats as much as possible.
• It's fine to pet your cats, but limit close contact (closer than 3 feet) to less than 30 minutes a day.
• Do not prepare their food, since you could potentially contaminate what they eat (same goes for your husband).

The length of time needed to continue these precautions will depend upon the dose administered, so you should talk to your physician about what he recommends for your husband, and then I'd follow the same rules for your cats.

For more information about Radiation Safety Recommendations, check out the information about radioiodine and how to reduce exposure after I-131 treatment on the American Thyroid Association website.

Can you make your cats radioactive?
The short answer is — no, you cannot make your cats radioactive unless they ingest food that you have contaminated. I would have your husband feed your cats, or if that isn't possible, you should wear gloves when preparing their food.

When you are close to your cats or your husband, you will be exposing them to the radioactive emissions coming from your body. These emissions are gamma rays, one of the types of electromagnetic radiation, and are similar to x-rays. All electromagnetic radiation travels at the speed of light (or approximately 300,000,000 meters per second), so your cats will not be able to outrun this radiation.

Fortunately, if you step back from your cats, these gamma rays will not be very strong and they will not reach your cats. Even if your cats are too close to you and are exposed to your emmissions, they will NOT retain the radiative waves that they received by being close to you  — again, once you step away, the gamma rays continue traveling at the speed of light until they dissipate.

It's like going to the beach on a nice sunny day. We receive radiation exposure from the sunlight that hits our exposed body during the day, but as soon as the sun goes down, we stop receiving radiation exposure. In this case, you, as the patient treated with the radioiodine, are acting like the "sun." We don't become radioactive or excrete radiation after spending a day at the beach, and neither will your cats after being close to you for a few minutes each day.

References:

1. Zanzonico PB. Radiation dose to patients and relatives incident to 131I therapy. Thyroid 1997;7:199-204.  
2. Marriott CJ, Webber CE, Gulenchyn KY. Radiation exposure for 'caregivers' during high-dose outpatient radioiodine therapy. Radiat Prot Dosimetry 2007;123:62-67.  
3. Sisson JC, Freitas J, McDougall IR, et al. Radiation safety in the treatment of patients with thyroid diseases by radioiodine 131-I: Practice recommendations of the American Thyroid Association. Thyroid 2011;21:335-346.  
4. American Thyroid Association (ATA) website. Radioiodine.