This is the third of three articles about hyperthyroidism in cats. It discusses the radioactive iodine treatment. See the first and second articles for discussion of hyperthyroidism itself and a summary of other treatments.
SURFEIT GETS RADIOACTIVE IODINE
Last Friday, Surfeit went to Miami Veterinary Specialists (MVS) to get radioactive iodine treatment for his hyperthyroidism. The treatment is administered by Dr. Erick Mears of I-Cat. MVS charges $1200 for the treatment. This includes the heart and abdominal ultrasounds as well as litter box liners and gloves to use after discharge (see below). It does not include follow-up labs, which need to be arranged with the cat’s primary veterinarian.
Dr Mears is based in Tampa, where he practices canine and feline internal medicine and performs radioactive iodine treatments. He travels to Miami, Cincinnati, and New York City to treat hyperthyroid cats. His partner in I-Cat treats cats in Fresno, CA. An interesting fact: Dr. Mears received I-131 treatment himself when he became hyperthyroid several years ago.
HOW THE RADIOACTIVE IODINE TREATMENT WORKS
As mentioned in a previous post, iodine is efficiently concentrated by the thyroid gland. I-131 is an unstable form (isotope) of iodine but is chemically identical to the more common stable iodine (127I or I-127). It is produced in nuclear reactors. Being unstable means that I-131 turns (decays) into another element with time. I-131 decays to Xenon-131 (Xe-131 or 131Xe, a non-reactive and therefore harmless element) within a few days to a few weeks (it has a half-life of 8 days). While decaying, I-131 emits beta particles that kill nearby tissue. Since the nearby tissue is mostly excess thyroid cells, we get the desired result of killing off enough of the gland so what’s remaining will not produce excessive amounts of hormone. If there is ectopic functioning thyroid tissue present, it will also take up the I-131 and be killed.
The following testing is recommended before I-131 treatment:
- A full set of blood chemistries, including T4, Cr and BUN
- heart ultrasound to assess heart function
Some veterinarians also recommend:
- abdominal ultrasound to screen for any medical issues which might make the cat not a good candidate for I-131 treatment.
- thyroid scintigraphy to define the amount of excess thyroid tissue and to evaluate for the possibility and extent of thyroid cancer. This can help in determining the amount of I-131 to administer.
Surfeit had the lab tests and heart and abdominal ultrasounds. The lab tests showed a high T4, of course, and liver enzymes and bilirubin still slightly high. We are thankful that both the heart and abdominal ultrasound were normal.
How It’s Done
The I-131 solution is given as a subcutaneous injection. Dr Mears calculates the amount to be given based on the cat’s T4 level. The cat is then placed in a special room, either alone or with other cats who have been given the same treatment. It is observed for any adverse effects of the injection. It stays in the hospital for a minimum of three days, until the radiation level has decreased to a level that is not considered hazardous to its human or animal housemates. Some centers will keep the cat longer. Visiting is not allowed and any blankets or toys you might have brought for your cat will not be returned. They will be disposed of in the hospital’s radioactive waste. I did leave a well-used towel and Surfeit’s favorite silvervine pillow to remind him of home during his hospital stay. There were four cats being treated at MVS this weekend and the facility can handle eight cats at a time.
What happens afterwards
The following are among precautions which should be observed for two weeks after discharge:
- Children and pregnant women should stay away from the cat and its litter box.
- The cat should not sleep with its humans or spend more than five minutes three to four times a day in close contact with them.
- The cat should not be allowed to walk on surfaces where food is prepared or go outside.
- Avoid contact with the cat’s saliva or foot-pads.
- Wash hands after all contact with the cat.
- Wear gloves when pilling the cat.
- Use a disposable litter box liner and flushable litter. Wear gloves when cleaning the litter box. Flush soiled litter down the toilet. If the cat makes a hole in the litter box liner, discard the litter box after the two weeks are up. Keep the litter box out of reach of children and dogs.
A tip from my own experience: Put a wee-wee pad on the car seat under the carrier when you take the cat home after the treatment. Surfeit peed in the carrier on the way home and I would have had a radioactive car seat had I not done that. The hospital gave me a cardboard carrier to take Surfeit home in and it got wet from the pee. I put it in an extra-sturdy garden trash bag and plan to keep it in the garage for three months before putting it in the trash. By then more than ten I-131 half-lives will have gone by and the radioactivity will be less than 1⁄1000th of what it originally was. I figure it should be safe to throw the box out by then. Maybe a friendly radiation physicist can tell me whether my reasoning is sound.
For me, the simplest thing was to put Surfeit in a separate room with his own litter box. That way I don’t have to switch all the litter boxes to flushable litter and don’t have to worry about the other cats getting radioactive stuff on their feet and tracking it all over the house. Your solution will differ if you don’t have an extra room to use. I also put lots of towels down on the floor in “Surfeit’s” room and left the bed covered with only a fitted sheet so I can easily clean up afterwards.
We are using a disposable litter box with liner and a wheat-based litter. So far the toilet has not complained. A litter scoop was provided to us by the hospital; if you are not so lucky, get a cheap litter scoop and throw it away when the two weeks are up.
If you need to take the cat to the vet before the two weeks are up, make the vet aware that the cat received I-131 and bring a copy of any discharge instructions you were given.
I found out an interesting fact about garbage disposal while discussing the above precautions with the vet tech. Apparently, many (maybe all) trash disposal facilities use Geiger counters to scan their trucks for radioactivity. If the counter registers excessive radioactivity in a truck’s load, an investigation is begun involving all sources along the truck’s route. So don’t be tempted to just throw all the litter into the trash or you could end up in no end of trouble. Airports security checkpoints also have radiation detectors, so take this into consideration if you are planning on traveling with your cat shortly after it’s treated.
The major risk of I-131 treatment is allergic reaction to the iodine or acute kidney failure, both rare. Some humans have also described fatigue, a burning in the throat, dry mouth and nausea after I-131 administration.
The cat may be lethargic and have a poor appetite for four to five days following the I-131 treatment. If the cat seems ill beyond this time or if there are other symptoms, you should call the vet who administered the I-131 or the cat’s primary vet. I’m glad to report that Surfeit is doing great and eating well. The only thing he’s unhappy about is being confined to one room and not being allowed to sleep with us.
The cat should be seen by a vet at most one month after the I-131 injection for a physical exam, blood work and urinalysis. (Our vet wants to see Surfeit two weeks after the injection.) Thyroid function will return to normal within a month following therapy in 85% of cats but it may take up to three months. A chemistry panel, T4 and urinalysis is recommended at two to four weeks and at three months following the I-131 treatment.
A single treatment with I-131 is effective in over 95 – 97% of cases but in 3 – 5% of cats a repeated injection is necessary for return to normal levels of thyroid hormones.
Back to Mizz Bassie
This end the series on feline hyperthyroidism and radioactive iodine treatment. I hope it’s been useful to my readers. There are many things mommy left out in the interest of not making this into a novel, so please ask questions if you have any. I will be updating everyone with Surfit’s progress as time goes on. Thank you for reading. Purrrrrrrrrrrrrrs.
SOURCES AND FURTHER READING
Wikipedia and other links above
Hyperthyroidism in Cats by Cornell Feline Health Center
The Merck Veterinary Manual, Merck & Co., Inc., 2005
Plumb, Donald, Plumb’s Veterinary Drug Handbook, Sixth Edition, Blackwell Publishing, 2008
Feline hyperthyroidism by Advanced Veterinary Medical Imaging
What Is Feline Hyperthyroidism? by Cat Thyroid Center
Erick Mears, DVM, personal communication