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Information About Feline Hyperthyroidism

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Semi-Requisite Disclaimer

No, I'm not a vet. I'm a guy who has lived most of his life with cats and who has watched too many die sooner than they should simply because there was something that I didn't know. The information and opinions here are not intended to replace working with a vet, and if you have a hyperthyroid cat (or think you might), I strongly suggest that you join one of the groups listed below. I will sometimes disagree with your vet, but I cannot guarantee that anything here will work for you. What I ask is that you do some real thinking. I'll give references and reasons, but those aren't enough. Check what I say. The vet has a degree, but that isn't enough, either. Check what the vet says. Cats are quirky, something you've probably noticed, and that can make diagnosing and treating them a combination of science, empathy, experimentation, and luck.

For More Help

The two groups listed below are almost certainly the largest accumulations of experience on the subject of feline hyperthyroidism and related problems. Some of the members have been working in the groups for many years, gathering information from many sources in addition to the results seen in the groups themselves. In addition, the people in the groups are largely those who will understand what you're going through, not the type to say, "just a cat". Even if things are going well on your hyperT adventure, I really suggest joining one of the groups. For what it's worth, I spend more time on the Facebook group but try to stay involved with both.

Facebook Hyperthyroid Cats Group Yahoo Feline Hyperthyroid Group

Feline Hyperthyroidism: Treatment Options

There are four options for treating feline hyperthyroidism, although two aren't really good choices. Below are descriptions of the options followed by a closer look at keys to using methimazole or the I-131 radioactive iodine.

The most common treatment is with methimazole or one of the related medicines. Next would be the I-131 radioactive iodine. Surgery was originally more popular and is still somewhat common outside of the US. The Hills Y/D iodine restricted or deficient food is essentially a last resort.

Methimazole and Related Medicines

Properly used, methimazole can be very effective, and a hyperT cat has a fair chance at living the same lifetime they would without the disease. However, take special note of “properly” because it's still too common to start on doses that are too high and to increase them too quickly. The details about using methimazole in any form (pills, liquid, treats, or transdermal) will be covered later in the essay.

Note that methimazole is the generic name in the US. Felimazole is a brand name and has a coating to hide the smell and taste. You'll also find methimazole called Tapazole or thiamazole, possibly other names. In the UK and some other countries, there's a liquid form called Thyronorm, but in the US, the medicine by that name is a treatment for hypOthryoidism in humans. Carbimazole is a related drug that is turned into methimazole in the body.

I-131 Radioactive Iodine

Ideally, this is the number one choice in terms of health because it's a cure. However, the up front cost is prohibitive for some people, and kidney or heart health may eliminate it as an option or at least complicate the process. More details below.

Surgical Removal of Thyroid Tissue

While this costs less than the 131, its success depends upon the skill of the surgeon who has to remove all the tumor, none of the healthy tissue, and avoid damaging the parathyroids. However, the biggest drawback may be the problem of ectopic tissue, which is tumor cells outside of the thyroid. The surgeon can't see such tissue, much less remove it.

Hills Y/D Iodine Restricted/Deficient Food

Admittedly, this food will lower the T4 in most cats if they will eat it. However, there are several problems that make this a last resort. The canned is mediocre nutritionally, while the dry contains essentially zero meat protein, making it a very poor cat food. While using Y/D a cat can have no other food or treats since they would counteract the effect. There is also no way to control how much or how little the food lowers the T4.

The biggest problem is the method itself. The food “works” by supplying an exceptionally low amount of iodine., making it impossible for the thyroid to produce as much T4. However, iodine is essential to cats the same as it is to humans and other animals. Based on earlier studies, the amount of iodine would be deficient for cats, but Hills commissioned a study that came up with a much lower iodine requirement. I am not sure if Hills has ever commissioned a study that didn't come up with the desired results. Long term side effects of iodine deficiency may include immune system malfunctions and an enlarged goiter.

Keys to Using Methimazole Well

Treatment with methimazole is mostly about dose. Since at least 2007, there have been formal sources supporting the 1.25mg twice a day starting protocol, and this is strongly confirmed by experiences within the online groups. (The 1.25mg protocol is specifed by L.A. Trepanier in “Pharmacologic Management of Feline Hyperthyroidism” published in Veterinary Clinics of North America: Small Animal Practices 2007 Jul;37(4):775-88, vii.) There is also disturbingly powerful documentation against using the 2.5mg twice a day protocol in most cases. What's more disturbing is that this documentation has actually been used to “support” the 2.5mg protocol. This will be more fully covered in The Dechra Dosing Dilemma. (Coming soon.)

The primary side effect of methimazole pills is digestive upset, which may range from just a little indigestion to vomiting and diarrhea. This may be slightly less with compounded liquid methimazole, but some cats dislike getting liquids even more than pills. Digestive issues are rare with the transdermal form (a gel/cream that's rubbed in the ear and absorbed into the bloodstream).

The second most common side effect is facial itching, but this is also fairly closely tied to starting dose. If the facial itching does happen, it can sometimes be controlled or limited with the use of ceterizine (Zyrtec) or another antihistamine. Talk to your vet about it. Control may require stopping the methimazole for a while and restarting on a lower dose. Cats will sometimes adapt to the med in a few weeks if the itching can be controlled long enough. However, the best way to avoid this problem is by starting with the 1.25mg protocol if possible. All of the side effects become more likely with higher starting doses.

Pilling a cat can be quite a trick. Check YouTube for suggestions and find what works best for you. Note that methimazole and other pills can be put into empty gel caps to hide taste and smell. You can usually fit several meds into one capsule if need be. The “size 3” is about the size of a Benadryl capsule. Size 4 is smaller. (The higher the number, the smaller the gel cap.) I'd avoid using anything larger than size 3 if possible. You can order empty capsules at or

Pill pockets are also an option, but cats often aren't impressed by the taste. Some people coat pills or gel caps with butter to get better results, and some have luck hiding the pill inside a bit of cheese, tuna, or a soft treat (which often work better than the pill pockets. Tomblyn Pill Masker is also an option. Any of the pills can be cut, even though the official stance is that you shouldn't do so. People have been doing it successfully for years, and fears about absorbing the med are essentially absurd.

Methimazole itself doesn't easily pass through the skin, which is why it has to be put into a transdermal cream to make it do so. (See the transdermal section below.) No, you shouldn't lick any powder off your fingers, and you should wash your hands afterward, but it's not a rush job. Pill cutters can be found at any pharmacy.

Any of the pills except Vidalta can be crushed as well. (Cutting Vidalta is okay, but crushing it eliminates the 24 hour release factor.) Some people crush the pill and put it into food, but the two problems are making sure the cat eats it all to get the full dose, and you risk putting the cat off their food by making it taste bad.

Lancelot was fairly easy to pill, but in other cases, I've found that Orijen “Original” treats work well. They are all meat, so they can also be given to diabetics, and most cats go nuts for them. Since they are a bit dry and crumbly, you may need to moisten your fingers slightly, but with a little patience, you can form the treat around the pill.

Always follow any pill with some food or liquid. Otherwise, the pill may stay in the throat and dissolve there or get tossed up a few minutes later in some secret location. Yeah, never trust a cat where pills are concerned.

Note that Vidalta is a 24 hour formulation of carbimazole, which the body converts into methimazole. Conversion rate is almost 70%, meaning 10mg of Vidalta is equivalent to 7mg of methimazole. This is a problem because the smallest Vidalta pill is 10mg, so even that is even higher than the 2.5mg twice a day protocol that's proven to be too much for the majority of cats. "Luckily", Vidalta may not absorb well and has been known to pass through the body completely undissolved. (People with sick cats get rather paranoid and aren't above searching through the litter pan.)

Using Liquid Methimazole

Methimazole can be compounded into a flavored liquid, and it can be easier to give in some cats, harder in others. The liquid does seem less likely to cause digestive upset, but it's still mostly about dose.....and making sure you get it all into the cat. Compounded medicines are still difficult to get in the UK, but methimazole in liquid is now available commercially. Be careful, because the name Thyronorm refers to a feline hyperthyroid medicine in the UK but is a human medicine to treat hypOthyroidism in the US. If pilling is an issue, Thyronorm may be a better option than transdermal in the UK.

Using Transdermal Methimazole

Overall, this is the form that I prefer, but it's not available (or easily available) everywhere in the world, and there are some tricks to using it.

1. I suggest insisting on 1ml syringes, not 3ml syringes (not well marked), not twist/click pens (hard to adjust dose, and I have reservations about reliability). With the 1ml syringes, you can control the dose and make very fine adjustments, which you can't do with pills or twist/click pens. Be aware that someone is pushing hard to make the pens standard, so you may have to fight to get the syringes, but it can be worth it.

2. I suggest a strength that will use .05ml per dose. For example, a strength of 2.5mg/0.1ml with a dose of .05ml equals giving 1.25mg of methimazole. Vets often still write the prescription to use 0.1ml per dose, but that amount of gel is messier and likely to increase residue, possibly decreasing absorption.

3.Request that the base be Lipoderm rather than one of the PLO (Pluronic Lecithin Organogel) options. Lipoderm has the best record for absorption and lowest likelihood of skin irritation.

4. You have to clean the ears regularly. How often depends on the amount of gel/cream used and the cat. Clean with warm water on a cotton ball (damp, not wet) at least every three days. Some people dose one ear and clean the other every time just to stay in a rhythm. When dosing or cleaning, be sure to support the back of the ear. Treat it all as a light massage.

5. You do need to wear something on your finger when applying the transdermal. Finger cots (cheaper online than at most pharmacies) are easiest overall, but some people cut the fingers off a glove since they can be washed and reused. Wearing a whole glove freaks out some cats. I suggest non-latex since some cats do have a latex allergy.

Remember that you need to retest 3-4 weeks after starting methimazole or changing the dose. The first time that you restest, you need to do the complete blood panel and CBC. Keep in mind that it's not just the numbers but how they relate to previous tests that matter. You're watching the creatinine in order to confirm kidney health, but there are also rare but serious allergic reactions that show up in an increased ALT (liver enzyme), or decreased WBC, or RBC/Hemocrit. Changes don't guarantee that there's a problem but mean you need to watch.

You need to get two consecutive results on the same dose in the ideal zone before stretching out the testing period. Ideal is about 1.5-2.5 on the U.S. scale or about 20-35 on the international scale. Once you get that, you stretch to three months. If still stable at three months, you can test every six months. Remember that treating with medicine or Y/D food can control the T4, but it doesn't affect the tumor that causes feline hyperT. The tumor continues to grow and require higher doses over time. How often increases are needed is unpredictable, which is one reason to retest regularly.

Keys to the I-131 Radioactive Iodine

Some people are afraid of this treatment just because of the word “radioactive”, but it's the simplest route and the best when finances and cat health permit. It's significantly more expensive up front, but in addition to having a healthy cat, you will likely recover the cost over 2-3 years through reduced vet and med bills. Unfortunately, not everyone can come up with that up front cost to begin with, but remember that methimazole is still a good option. If there is mild to moderate chronic kidney disease, that used to eliminate the 131 option, but many facilities now have better protocols, so it's still a possibility. Extreme kidney disease, serious heart issues, or cancer generally prohibit doing the 131.

The treatment itself is simply a single injection into the scruff (or some still use a pill) and then a brief stay at the 131 facility while the system works out the radiation. In the US, the average stay is 3-5 days, but it's still 10-14 days in some countries. Many people worry about their cat being away from home, but remember that you're trading a few days of stress (mostly yours) for the chance at years of tumor-free health.

Facilities give a rather long list of post-treatment “restrictions”, but the reality is that cats aren't allowed to go home until radiation is well below a safe level. Most people ignore all of the restrictions except litter care and maybe minimizing French kisses for a while. In other words, if you can resist eating out of the litter pan, everything should be fine. Oh, and no making chocolate chip cookies from the poop, either.

There are many differences among the 131 clinics. When getting a price quote, ask what's included and what will be required that isn't included. You'll usually need to get blood tests done within 30 days of the treatment, and some clinics require x-rays and other tests that may not be in the quote. (Some that appear higher based on the quote are actually including more.)

In addition to length of average stay, ask about post-injection care. Most clinics will intervene if a cat happens to get sick in the first few days (which rarely happens and has nothing to do with the 131), but there are some clinics that won't intervene under any circumstances the first day or two. Others give lots of direct attention and clearly love cats, not just money. You'll also find that some have regular cages, while others have multi-level “condos”. Some even have cat tv and other entertainment and cameras that allow you to check in from home.

"Alternative" Treatments

I strongly believe in herbs, acupuncture, and a variety of other treatments for many problems, but feline hyperthyroidism isn't one of them. Yes, some of the side issues, such as digestive problems, can be treated with low risk options, but treating the disease itself with something such as Thyroid Support Gold will only hide the problem. It may reduce symptoms, but it doesn't control the T4. So while the human happily thinks the cat is doing well, in reality the cat is getting sicker as the T4 continues to rise. By the time it's bad enough to show, the cat may be in very serious condition.


We currently have quite a few fur-kin that we care for or help care for. All were strays at one time, and some have extra issues with one asthmatic, one FIV positive, one FeLV positive, and one hyperthyroid. The information and stories are free to all, and that especially applies to the medical material since I don't want to be accused of practicing without a license, but donations are certainly appreciated.

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