Surfeit Gets Radioactive Iodine

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.


Miami Veterinary Specialists

Miami Veterinary Specialists entrance

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-​​CatMVS 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.


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.

Preliminary Testing

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

Surfeit with Dr. Mears

Surfeit with Dr Erick Mears, who gave him the I-​​131 treatment.

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 11000th 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.

Surfeit's injection site

I couln’t get any decent pho­tos of Surfeit because he’s so lonely, he just wants to be close to me when­ever I’m there. So here’s a photo of his injec­tion site. It’s shaved to watch for any reac­tion to the injec­tion. Surfeit had none.

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.

Treatment Failures

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.



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

I-​​Cat website

Insights into Veterinary Endocrinology blog on iopanoic acid

Erick Mears, DVM, per­sonal communication

20 thoughts on “Surfeit Gets Radioactive Iodine

  1. Thanks so much for the series. This will be such help­ful infor­ma­tion for any­one about to have to do the same pro­ce­dure to their cat or dog. I per­son­ally know some­one here who had to do that for their cat. It was totally suc­cess­ful, and the cat lived for another 5 years. We wish the same for Surfeit!

    • Oh, Surfeit wishes it were over, too. He wants to go back to his old life of sleep­ing with the humans. He prob­a­bly even misses being chased by me, MOL! Purrrrrrrrrrrrrrs.

    • Well, I don’t think it’s all that bad. The good thing is that the I-​​131 can harm you only if taken inter­nally. If you wash your hands often, you should be OK. The radioac­tiv­ity just doesn’t pen­e­trate that far (only 1/​4″ at most) and it dis­si­pates over time, so it will be mostly gone in a few months. Still, it’s best to be care­ful so you don’t have to worry about it.

      As for hav­ing the extra room, we know we are very, very lucky about that. It makes things so much easier.

      • Agreed! And it also dis­si­pates over dis­tance, fol­low­ing the inverse square law — which means that if you dou­ble the dis­tance, the radi­a­tion level drops to 14 the measurement.

    • Thank you, Sparkle. We hope Surfeit’s thy­roid goes back to nor­mal after this as well. And yes, the more you learn about some­thing, the more you find out how involved it is. We love learn­ing. Purrrrrrrrrrrrrrs.

  2. Hi Mizz Bassie! We gonna let mommy talk now cuz we don’t do math ‘n’ stuff, MOL!

    (thanks kids) I’m so used to deal­ing with radi­a­tion ther­apy in humans that I didn’t con­sider that of course a cat would get the I-​​131 via sub-​​q!

    And we’re guess­ing that Surfeit received around a 4 mCi dose? If so, then he’s already almost at back­ground radi­a­tion level — if not there already.
    Marty’s not even sure he could pick any­thing up with the Geiger counter (though on a swipe test pos­si­bly). And I also assume the gloves you were sent home with are lead-​​lined … just as a pre­cau­tion. (Are they a lovely orange color? They’re nice thick gloves, great for scrub­bing pots & pans with ;-)

    You’re prob­a­bly fine to throw the pad out now. The rea­son that he was released in just a few days is that bio­log­i­cal half-​​life (the time it takes the body to elim­i­nate half of the I-​​131) is shorter than the stan­dard 8-​​day I-​​131 half-​​life. (Standard prac­tice is to wait until the read­ings reach T1/​2E for sur­face emis­sions before the cat is released — some­where between 2.19 and 4.70 days.)

    If you want to play it safe, just wait another half-​​life — 8 days or so — and then dis­card. No need to hang on to it for 3 months! We have some old Fiesta ware plates that Marty uses as a demon­stra­tion to his stu­dents that prob­a­bly reg­is­ters higher than Surfeit now. (Though they’re α emit­ters, not ß or gamma)

    Also thought you might find it inter­est­ing that human patients Marty treats receive between 30 and 200 mCi — and they usu­ally go home within a few days.

    But because all med­ical (human and vet) prac­tices that uti­lize radioac­tive mate­ri­als are under the scrutiny of the NRC, nat­u­rally your vet is going to err *far* on the side of cau­tion. As well her should!

    BTW, Floridians are exposed to around 200 mrem/​year in back­ground radiation.

    • Surfeit did get 4 mCi, you are so right. Glad to know all the pre­cau­tions are to be extra cau­tious. Mommy will be sure to hug Surfeit more, the poor lonely thing.

      NO, no lead-​​lined gloves. They do sound won­der­fully use­ful and we love bio­haz­ard orange! We have just the usual dis­pos­able med­ical gloves. I think they want you to wear the gloves so you don’t get the I-​​131 on your hands and then in your mouth. Of course, you are sup­posed to wash your hands after con­tact with the “hot cat” but you know how that goes. Mommy read that low doses of ingested I-​​131 are actu­ally more dan­ger­ous that high doses because they don’t kill the thy­roid cells out­right and might allow some with cancer-​​causing muta­tions to sur­vive and cause prob­lems later on.

      It’s easy to get all scared of radi­a­tion because it’s a poten­tial dan­ger that you can’t detect with your senses and because many peo­ple don’t under­stand the bio­log­i­cal effects and what they really need to worry about. Good to know we don’t need to keep the garbage around for three months, the garage is junked-​​up enough as it is. Purrrrrrrrrrrrrrs.

  3. OMC! That sounds like scary stuff. I guess if one of us had to go through that, we’d be in iso­la­tion in one of the bed­rooms. I would be hor­ri­ble cuz we all like to be with Mom when she’s home. The iso­lated kitty would be so miserable.

    Did Surfeit cry alot at being in isolation?

    • Radioactive iodine is not all that scary, as Maxwell, Faraday’s and Allie’s mom points out. All the pre­cau­tions are just to be extra safe, so no humans get any radioac­tive iodine inside of them. It really can’t hurt you unless you eat it.

      Yes, poor Surfeit kept meow­ing and scratch­ing at the door the first day he was by him­self. He’s resigned him­self now, but I know he’ll be thrilled to be out of iso­la­tion and back to his nor­mal rou­tine. Just 10 days to go. Purrrrrrrrrrrrrrs.

  4. Pingback: Mizz Bassie on Hyperthryroid Treatments VBP | Dash Kitten Cat Blogger from Middle Earth

  5. Hi Oker, good to hear from you. Thank you for ask­ing abu­out Surfeit. He’s doing great. His heart rate is nor­mal, he’s put on weight, his coat looks bet­ter and his kid­ney func­tion has remained good. Yay! Mommy found a lump on his back, though. It was a mast cell tumor on nee­dle aspi­ra­tion, so he had surgery to remove it on Thursday. The vet thinks he got it all and the inci­sion looks great, but Surfeit hates the donut he has to wear around his neck. I’ll have to put a photo of him in it on the blog :)

    • Aaawww! Poor baby! Happy that your mom seems to have every­thing under con­trol! May be Surfeit would be much hap­pier with a jump­suit instead of the donut? They have these for cats and dogs… but you prob know that already. ;-) Give him a hug from me. Will keep paws crossed that all will be alright very soon! <3

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