Tag Archive | Triiodothyronine (T3) in Cats

Kitty Corner: Feline Hyperthyroidism

Mister C_Portrait_Lightened_2013_JBowman In a temporary departure from human concerns, today we take a brief look at a health issue for older cats—overactive thyroid. 

I need to decide whether to keep my little buddy on meds or take him in for a thyroidectomy.What would you do? Cast your vote.

ALSO SEE: Kitty Corner: Feline Hyperthyroidism – Update


Just before my husband and I went to Vancouver and Seattle in June, I took myself to the gynecologic oncologist (see my latest post) and my buddy Mister C to the vet. While away, I received calls from both doctors.

Since last year, Mister C had lost almost a pound, yet he seemed to be hungry all the time. One of his cues to eat is when I enter the house, which, often for coincidental reasons, is just around his dinnertime. But recently, no matter when I left and returned, he thought my arrival meant chow time.

I casually mentioned this to the vet when I took him in for his annual exam, and her interest perked up. After a short exam and discussion, she said that older cats—Mister C just turned 14—often develop overactive thyroid (hyperthyroidism). She explained that cats have two thyroid glands, and sometimes only one is enlarged, as it is in Mister C’s case (likely from a benign tumor called a thyroid adenoma). The increased thyroid hormones, thyroxine (T4) and triiodothyronine (T3), but primarily T4, were ramping up his metabolism so that not only was he eating more, he was losing weight. (Oh, for that problem! My situation is directly opposite—I’m hypothyroid, like many women of a certain age.) In addition, his heart rate was too fast (tachycardia).

So, out came the shaver, a syringe, and an assistant to hold Mister C’s head still while the vet shaved his throat and drew a blood sample. The test results came back quickly (unlike for us humans, as I have described in my recent posts) and showed increased hormone levels and also increased platelets. Because I was away at the time and came home on a holiday, the vet left his medication in a pickup box, and I started Mister C on Felimazole (methimazole) 5 mg tablets twice a day on July 4.

Felimazole 5 mg

It hasn’t always been easy getting the pills down him. They are bright orange and, although they are coated, probably have a bitter taste. After finding pills carefully removed and lying next to his bowl, or licked clean of their orange coating but remaining in his food, I finally found that taking one can of Friskies paté and embedding a pill into each serving (now a third, formerly a half) near the top seemed to work most of the time. His appetite markedly decreased, and he was gaining weight (both a good and a not-so-good thing, I guess).

We returned to the vet last week for a repeat blood test and got the results the next day—decreased thyroid hormone levels and decreased platelets, which is good. And he gained about 3/4 of a pound, which the vet said is good (it just isn’t pretty as the skin sort of dangles in the wind). Plus, he had a regular, normal heart rate.

So now comes the treatment decision. Not treating him isn’t an option as the disease would be fatal, leading to heart and possibly kidney disease. Because his metabolism has adjusted to the medication over the past six weeks, he’s ready for either more pills or surgery if we decide to go that route. (Some sources cite other options, such as radioactive iodine, but our vet didn’t offer anything but meds or surgery. Also, after scanning through the resources listed below, I started to become a little more alarmed and a little more confused, but I offer them here as objective sources of information.)

I asked the vet to explain the risks and benefits of each modality (as well as costs), and this is what she said:


  • Medication manages the problem, but doesn’t cure it; so he would need pills twice a day for the rest of his life (they cost about $35 per Rx)
  • Repeat blood tests would be needed every six months to monitor the thyroid hormone and platelet levels (the tests cost about $200 a pop)
  • The medication wears off after about 10 hours, and during the interim before the next dose his heart and blood pressure could be stressed, possibly leading to heart disease (probably congestive heart failure) and decreased longevity


  • Surgery “cures” the problem, leading to possibly increased longevity; a unilateral thyroidectomy (removal of only one gland) requires a one-night stay and costs about $400; a bilateral procedure (removal of both glands) requires a two-night stay and costs about $600
  • After a bilateral thyroidectomy (if he needs to have both glands removed), he would have to stay the extra night to have his calcium levels checked because the tiny parathyroid glands could be removed accidentally during surgery (100:1 chance); if this happened, he would need to take calcium supplements for the rest of his life to prevent seizures and other problems
  • A repeat blood test would be needed about a month after surgery to check his thyroid hormone levels (another $200); even with both glands removed, cats usually do not need supplemental thyroid medication (unless too much tissue is removed) because some thyroid hormones are produced by other tissues and glands (I will be looking into this a little more)

So, it’s decision time. The vet said that her clients’ decisions are split 50-50. We have that exact split in our household—I favor surgery (despite the costs) to take care of the problem once and for all, and my husband favors continued medication to spare Mister C the trauma of surgery (if it is indeed a trauma—it may not be—and despite the costs, which may in time become equivalent to the surgical costs). The vet herself recommends surgery.

We’re running out of pills, so I need to make a decision pretty quickly.

What would you advise Mister C to do?


Advanced Veterinary Medical Imaging (AVMI) – Feline Hyperthyroidism

American Society for the Prevention of Cruelty to to Animals (ASPCA) – Hyperthyroidism

CatInfo.org – Feline Hyperthyroidism

Cornell University College of Veterinary Medicine – What is hyperthyroidism?

Feline Hyperthyroid Treatment Center (FHTC) – What is feline hyperthyroidism?

VCA Animal Hospitals

– Hypocalcemia or Low Calcium Levels in Cats

Thyroid Hormone Testing in Cats

Washington State University College of Veterinary Medicine – Hyperthyroidism in the Cat

WebMD – Hyperthyroidism in Cats