Cat Diseases & Illnesses - Hyperthyroidism - Felimazole


What is Felimazole?

Felimazole is a drug used in the treatment of an over active thyroid in cats. This disease is also called hyperthyroidism. Felimazole contain the active ingredient methimazole which is also known as thiamazole and is manufactured by Dechra Veterinary Products. It is available as pots of 100 tablets, either pink 2.5mg or orange 5mg tablets. Felimazole is given orally once or twice daily. The exact dosing regime will be determined by your veterinary surgeon.

What is Felimazole used for?

Felimazole is used in the treatment of hyperthyroidism in cats. This is a condition when the thyroid gland is producing too much thyroid hormone. Felimazole blocks part of the pathway involved in making this thyroid hormone and so reduces the amount of thyroid hormone circulating in the body. Thyroid hormone, also called thyroxine, up regulates the metabolism causing a faster heart rate, an increased appetite and weight loss amongst other signs. Giving Felimazole at a dose that suppresses the hormone will reduce these signs.

Felimazole tips

·       Felimazole is only for use in cats

·       Felimazole is available in two sizes; 2.5mg and 5mg

·       Felimazole tablets are small and tasteless to make it easier to give them to your cat

·       You should only give Felimazole if directed to do so by a vet

·       The dose of Felimazole will be determined by your vet

·       Felimazole can be given with food

·       Felimazole can be given once or twice daily

·       If you suspect an overdose of Felimazole, stop treatment and contact your vet as soon as possible.

·       If you see side effects associated with Felimazole then call your vet for advice

Side effects

·       The most common side effects are vomiting and going off the food and pruritus (itching)

·       Felimazole should be used with care in cats with existing kidney disease

·       Felimazole can also have effects on the liver when given long term

·       Felimazole should not be used in pregnant or lactating queens

·       Side effects are often mild and temporary and do not require the treatment to be stopped

·       In more severe cases, most side effects will resolve after stopping treatment

·       Women of childbearing age should wear gloves when handling the litter of cats treated with Felimazole

·       Felimazole can have effects on the white blood cells

·       Regular blood tests are required for cats taking Felimazole for monitoring

·       If you think you see any side effects then contact your vet for advice

Dosage and Administration

·       Felimazole comes as small, round tablets in two sizes.

·       Felimazole 2.5mg tablets are pink and the 5mg tablets are orange

·       Felimazole tablets are given orally with food

·       The dose will depend on the level of thyroid hormone in your cat

·       The standard starting dose of Felimazole is 2.5mg twice daily

·       This dose is then tailored to suit each patient according to blood tests

·       The total daily dose should be divided into two where possible

·       The aim should be to give the lowest possible dose of Felimazole that will control the problem,

·       Do not alter the dose of Felimazole without speaking to your vet first

·       Regular monitoring is essential to check your cat’s progress

Further information about Felimazole

Felimazole is a drug used to control hyperthyroidism in cats. This is when the thyroid glands, which are situated in the neck, start to produce too much thyroid hormone. Thyroid hormone is also called thyroxine (T4) and triiodothyronine (T3).  Felimazole contains the active ingredient methimazole, also called thimazole.

Thyroxine and triiodothyronine are made of a molecule called thyroglobulin combined with iodine. Felimazole blocks this reaction and so suppresses the formation of further thyroid hormone. Thyroid hormone up regulates the metabolism and causes a wide range of clinical signs such as an increased heart rate, sometimes a heart murmur, increased appetite, increased thirst and weight loss. Giving Felimazole reduced all these clinical signs.

Medical management with Felimazole is one of three ways to treat hyperthyroidism. It is possible to irradiate the thyroid gland with radioactive iodine but this is a specialist procedure that is usually done at a referral hospital and cats are hospitalised for up to a month. The thyroid gland can also be removed surgically but Felimazole is involved in this treatment plan too as it is recommended to stabilise these surgical candidates with Felimazole for three weeks prior to surgery to normalise the levels of thyroid hormone as this makes them safer anaesthetic candidates.

Felimazole is manufactured by Dechra Veterinary Products and is available as pots of 100 tablets of either 2.5mg or 5mg. The dose will be determined by your vet and Felimazole can be given once or twice daily by mouth.

Further Information about dosage and administration

As mentioned above, Felimazole comes in two different sizes. The 2.5mg tablets are pink in colour and sugar coated. They are small and round to make them easier to administer to cats. The 5mg tablets are also round in shape and sugar coated but are orange in colour. They are designed to be swallowed whole and tablets should not be split. They can be given with food or directly into your cat’s mouth.

The standard starting dose of Felimazole is 1 x 2.5mg tablet twice daily, morning and night. This is the same whether you are embarking on long term medical management of hyperthyroidism with Felimazole or just using it in the short term prior to surgery. When using Felimazole for long term medical management, blood tests will usually be taken after three weeks of treatment to measure the levels of thyroxine (T4) in the bloodstream. Other bloods tests will sometimes be run at the same time looking at things like kidneys and liver parameters as well as red and white blood cell levels. The dose of Felimazole will then be titrated so as to achieve the lowest effective dose of Felimazole.

If the thyroxine levels are still above the normal range then the dose of Felimazole will be increased. If conversely they have fallen to below the normal range then the dose of Felimazole will be decreased. The dose is usually adjusted by 2.5mg increments. Ideally the total daily dose of Felimazole is divided in two and half is given in the morning and half in the evening. You can give the whole dose in one go in cats who may be more difficult to tablet but it is likely not to work as well as twice daily dosing.

Regular monitoring of thyroxine levels is essential to check your cat’s progress and to make sure they are receiving the correct dose of Felimazole.

Frequently Asked Questions

Q. What does Felimazole do?                                                                                                                                               

A. Felimazole is used to treat an over active thyroid gland in cats. It blocks production of the thyroid hormone, thyroxine.

Q. How is Felimazole given?                                                                                                                                                     

A. Felimazole tablets are given orally and can be given with or without food.

Q. Can I break up the Felimazole tablet to make it easier to hide in food?                                                                  

A. No, Felimazole tablets are designed to be swallowed whole, not to be split.

Q. How long will my cat need to take Felimazole?                                                                                                                  

A. This depends. Felimazole can be given long term to control hyperthyroidism or just for three weeks prior to surgery to remove the thyroid gland.

Q. Are there any side effects associated with Felimazole?                                                                                                     

A. There can be side effects seen in a proportion of cats. The most common are vomiting, and going off their food. If you see any side effects you should call the vet for advice.

Q. How often will be pets need rechecks at the vet while taking Felimazole?                                                                                    

A. When on long term treatment with Felimazole, you will need an initial recheck after three weeks and then regularly after that at your vet’s discretion. There will be blood tests involved in these rechecks.

Q. I have given too many Felimazole tablets by accident, what should I do?                                                                             

A. You should stop treatment and call your vet for advice as soon as possible.

Q. My cat seems to have started losing weight again despite being on Felimazole, should I give more?

A. No, you should book an appointment for a recheck with the vet so they can check the thyroxine levels. Do not change the dose without checking with the vet.

Q. What would happen if i did not treat my cat with Felimazole?                                        

A. If left untreated, hyperthyroidism can lead to extreme weight loss and signs of heart failure amongst other things. There other options for treatment though such as irradiation and surgery.

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