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