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

Hyperthyroidism in Cats, continued: Treatment


The sec­ond of three posts on hyper­thy­roidism in cats. The first post dis­cusses the func­tion of the thy­roid and the causes and effects of hyper­thy­roidism. The third post will dis­cuss radioac­tive iodine treatment.




Hyperthyroidism may be treated by med­ica­tion which sup­presses thy­roid hor­mone pro­duc­tion, by sur­gi­cally remov­ing part of the thy­roid gland, by feed­ing a diet defi­cient in iodine or by admin­is­tra­tion of radioac­tive iodine.

Tapazole (methi­ma­zole) is the only med­ica­tion rec­om­mended for treat­ment of hyper­thy­roidism in cats. White blood cell and platelet dis­or­ders occur in 15% of treated cats but usu­ally dis­ap­pear with­out treat­ment or the need to dis­con­tinue the med­ica­tion. 10% of cats have vom­it­ing, poor appetite and lethargy. This is also gen­er­ally tem­po­rary. Rare (below 5%) side effects that require stop­ping the med­ica­tion include facial scratch­ing, bleed­ing, liver prob­lems, low platelets and ane­mia caused by destruc­tion of the red blood cells by the immune sys­tem (immune hemolytic ane­mia). Surfeit is one of the rare kit­ties that get liver prob­lems with Tapazole. His T4 became nor­mal but his biliru­bin went up and so did his liver enzymes. He had to stay in the hos­pi­tal on IV flu­ids overnight and we are now giv­ing him SQ (sub­cu­ta­neous) flu­ids every other day. The good news is that his kid­ney func­tion did not get worse when his T4 got bet­ter, so we are hop­ing it will stay good after the I-​​131.

Iopanoic acid is an X-​​Ray con­trast mate­r­ial that sup­presses the thy­roid as a side effect. There is very lit­tle expe­ri­ence with its use in cats and what there is does not sug­gest it’s a real­is­tic long-​​term solution.


Surfeit show­ing off his “French Poodle” front legs, cour­tesy of his stay at the ani­mal hospital.

Surgical removal of part of the thy­roid gland (thy­roidec­tomy) is an effec­tive treat­ment of hyper­thy­roidism but there are sev­eral caveats. One is, as with all surgery, the risk of anaes­the­sia. The sec­ond is that some­times there is func­tion­ing thy­roid tis­sue located  in the else­where in the body. This ectopic tis­sue is usu­ally in the chest but can be any­where and can­not be found on rou­tine thy­roidec­tomy. Thus some­times enough thy­roid tis­sue is left after the surgery for the cat to con­tinue to be hyper­thy­roid. Finally, if the sur­geon removes most of the thy­roid gland, there is the risk of acci­den­tally remov­ing or caus­ing injury to the parathy­roid glands, which would cause major prob­lems. For all those rea­sons, sur­gi­cal removal of the thy­roid is no longer the treat­ment of choice in most uncom­pli­cated cases.

Hill’s Prescription Diet y/​d Feline Thyroid Health is a new med­ical food for cats which con­tains a reduced amount of iodine. Since all thy­roid hor­mones con­tain iodine (see the dia­grams in the pre­vi­ous post), if you don’t give the cat iodine, it can’t pro­duce thy­roid hor­mone and it won’t be hyper­thy­roid. (The y/​d has a pre­cisely con­trolled amount of iodine so the cat can make just enough but not too much thy­roid hor­mone.) The trou­ble is that if the hyper­thy­roid cat gets any other food, the ben­e­fit of the med­ical food is negated. So no treats, no snack­ing from other cats’ bowls, no hunt­ing for out­door cats. That’s a dif­fi­cult man­age­ment prob­lem for multi-​​cat house­holds. For us, it wouldn’t be prac­ti­cal because my fursib Java has aller­gies and we all eat a veni­son and pea hypoal­ler­genic food because of that. There are also con­cerns about the nutri­tional value of y/​d as the sole source of nutri­tion for a cat.

Radioactive iodine treat­ment is the method of choice for treat­ing hyper­thy­roidism in cats. It works because the thy­roid gland is very effi­cient at con­cen­trat­ing iodine from the blood­stream (just look at all those iodine atoms in thy­roid hor­mones), so when you give radioac­tive iodine, it goes right to the thy­roid and kills the cells by its radi­a­tion. The treat­ment does not affect other organs because the thy­roid fishes all the iodine out of the blood stream and holds onto it.

To be continued…

The next post will dis­cuss radioac­tive iodine treat­ment: how it’s done and what to expect afterwards.



Wikipedia links 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

http://​www​.endocrin​evet​.blogspot​.com/ — var­i­ous articles

Erick Mears, DVM, per­sonal communication

Surfeit Is Hyperthyroid

I got mommy to write this arti­cle because one of my fursibs is hyper­thy­roid and I thought it was a good oppor­tu­nity to write about the hyper­thy­roidism and its treatment.



Surfeit at the food bowl

Surfeit is the senior cat we adopted recently. We were told he was eleven years old. At his first vet visit he checked out fine but the labs showed a bor­der­line T4. The vet ordered a free T4 and that was high, demon­strat­ing that Surfeit is hyper­thy­roid. We were sur­prised because he wasn’t what you think of as the rav­en­ous hyper­ac­tive cat. He’s actu­ally not a very good eater and we’ve had to give him Periactin (cypro­hep­ta­dine) to improve his appetite. He was started on Tapazole (methi­ma­zole) but blood tests done a month later showed that the med­ica­tion was affect­ing his liver and it had to be stopped. This means that radioac­tive Iodine, com­monly called I-​​131 (131I in proper sci­en­tific nota­tion) is the only prac­ti­cal option for him.


T3 or triiodothyronine

T3 or tri­iodothy­ro­nine. Note the three iodine atoms.

T4 or thyroxine

T4 or thy­rox­ine. Note the four iodine atoms.

The thy­roid gland is located in the neck. In humans it lies just over the bump below the Adam’s Apple. When your doc­tor puts a hand on your neck and tells you to swal­low, he or she is check­ing whether your thy­roid is enlarged. The thy­roid pro­duces and secretes thy­roid hor­mone, which is actu­ally a mix­ture of sev­eral hor­mones. The major thy­roid hor­mones are T3 and T4, con­tain­ing three and four iodine atoms, respec­tively. Thyroid hor­mones travel in the blood stream bound to spe­cial pro­teins. They are taken up by cells and help reg­u­late many dif­fer­ent cell func­tions includ­ing the body’s use of pro­tein, fat and car­bo­hy­drate. The usual blood test for T4 mea­sures total T4, includ­ing T4 that is bound to its trans­port pro­tein and the unbound (free) T4. The spe­cial free T4 test is more accu­rate because only the unbound T4 is active.

The thy­roid is reg­u­lated by the hypo­thal­a­mus and the  pitu­itary gland via TRH (thyrotropin-​​releasing hor­mone) and TSH (thyroid-​​stimulating hor­mone or thy­rotropin). When the hypo­thal­a­mus senses that there is not enough thy­roid hor­mone in the body, it secretes TRH, which stim­u­lates the pitu­itary to secrete TSH. TSH causes the thy­roid gland to pro­duce thy­roid hor­mone. When there is enough thy­roid hor­mone in the body, the hypo­thal­a­mus stops secret­ing TRH and the pitu­itary, in its turn, stops TSH secre­tion. This sys­tem of con­trol is called the hypo­thal­a­mic – pitu­itary – thy­roid axis.



The typ­i­cal hyper­thy­roid cat is under­weight in spite of a good appetite. It’s hyper­ac­tive and may have exces­sive uri­na­tion, vom­it­ing and diar­rhea as well as an unkempt appear­ance. The thy­roid may be pal­pa­bly enlarged. There may be a heart mur­mur, increased heart rate, abnor­mal heart rhythm and heart fail­ure. The heart fail­ure is gen­er­ally reversible once the hyper­thy­roidism is treated but the cat might require med­ica­tion in the mean­while. Blood flow to the kid­neys is increased, and this may mask pre-​​existing kid­ney dis­ease, which becomes appar­ent once the hyper­thy­roidism is con­trolled. Blood pres­sure may be increased and this may dam­age many organs, includ­ing the kid­neys, eyes, brain and heart. A few cats have what is called “apa­thetic hyper­thy­roidism,” with decreased activ­ity and appetite. This is the cat­e­gory Surfeit falls into. He’s actu­ally a poor eater and has required appetite-​​stimulating medication.


Hyperthyroidism most com­monly occurs in middle-​​aged cats and is typ­i­cally caused by a benign tumor (ade­noma) of the thy­roid gland. A minor­ity or hyper­thy­roidism in cats is caused by can­cer­ous (malig­nant) thy­roid tumors. Some cases must be caused by prob­lems with the pitu­itary or hypo­thal­a­mus but I haven’t come across a men­tion of that so it must be very rare indeed.

To be continued.…

Mommy will be post­ing about the treat­ment of hyper­thy­roidism in cats and the radioac­tive iodine treat­ment in the next few days.


Sources and Further reading

Wikipedia 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