Hyperthyroidism in Cats, continued: Treatment

HYPERTHYROIDISM IN CATS, continued

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.

Treatment

Surfeit

Surfeit

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

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.

 

SOURCES AND FURTHER READING

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