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