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Client Factsheets > Cats > Medication: Methimazole (thiamazole) 

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CONTRIBUTOR(S): Vetstream Ltd, Gigi Davidson, Špela Oberstar, Fear Free,

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Medication: Methimazole (thiamazole) 

Your cat has been prescribed methimazole (also known as thiamazole) for the stabilization of hyperthyroidism prior to thyroidectomy or for long-term control of hyperthyroidism.

ℹ️Key points: • Antithyroid. • Do not use in pregnant or lactating cats. • Do not use in cats with primary liver disease, diabetes or autoimmune disease. • Do not use in cats with white blood cell disorders or platelet disorders such as thrombocytopenia. • Do not crush or break tablets. • Always wash hands after handling tablets. • Dosage: twice daily before stabilization or once daily if used long-term. 

⚠️See specific warnings below for pregnant women and women of child-bearing age. ℹ️This information sheet is designed to help answer commonly asked questions and help you give the medication safely and effectively to your pet. It does not contain all available information for this medication. If you have other questions or need more information, contact your veterinarian or pharmacist. ⚠️Always follow the dosing instructions provided by your veterinarian.

Name, Brand names, Class of medication and Storage

Name

  • Methimazole (me-thi-ma-zole).

Brand name

  • Felimazole® (UK, Europe & US).
  • Felidal ® (UK & Europe).
  • Normazole ® (UK & Europe).
  • Thiafeline ® (UK & Europe).
  • Thiamacare ® (UK & Europe).
  • Thiamapet ® (UK)
  • Thiamatab ® (Europe)
  • Thyrasol ® (UK & Europe)
  • Thryonorm® (UK & Europe)

Class of medication

  • Antithyroid. Controls excessive production of thyroxine (T4) and triiodothyronine (T3).

Storage

  • Store as directed at room temperature and in a dry environment.

Why has my cat been prescribed methimazole (thiamazole)?

Your veterinarian has prescribed methimazole for the stabilization of hyperthyroidism prior to thyroidectomy or for long-term control of hyperthyroidism.

How is the medication given?

Methimazole (thiamazole) is given orally as coated tablets or as an oral solution. An unlicensed option in the form of a transdermal gel (administered to the cat’s inner ear) is available through compounding in some localities and has been proven to be effective in some cases. Unlicensed medications are only appropriate when licensed options are not tolerated. Your vet will discuss this option with you, and the implications of using unlicensed preparations, if they feel this is appropriate for your pet.

Are there any side-effects or drug interactions?

The following side-effects have been reported:

  • Anorexia/inappetence.
  • Vomiting and gastrointestinal effects.
  • Lethargy.
  • Abnormal vocalization.
  • Hematological abnormalities.
  • Jaundice.
  • Immune side-effects
  • Facial excoriations (sores) in cats who are allergic to methimazole. Contact your veterinarian immediately if you notice excessive scratching in the facial area.
Drug interactions can occur with:
  • Phenobarbitone: can decrease efficiency of methimazole.
  • Some interactions have been reported with certain deworming medications (benzimidazoles) and with proton pump inhibitors such as omeprazole.
  • Anticoagulants (heparin, warfarin, rivaroxaban) may be potentiated by methimazole.

How long will my cat need to receive this medication?

Methimazole (thiamazole) is administered for life to control hyperthyroidism. Methimazole should always be administered according to your veterinarian’s instructions.

How should I dispose of this medication if I don’t use it all?

Dispose of any unused methimazole (thiamazole) safely - do not flush down the toilet or sink. Ask your veterinary team if they take back medication for disposal. Do not save methimazole for future use or give it to other pets. ⚠️Owners should wear impermeable gloves when handling this medication and the litter of treated cats. Hands should be washed after medicating the cat. This advice is especially important for pregnant women or those of child-bearing age, as this medication can affect the unborn fetus. The off-license use of gel preparations that are applied to the skin carries additional risk to pet owners, particularly pregnant women, and these risks should be discussed with your vet before commencing such treatments.

What if I miss a dose or give an extra dose?

If you miss a dose, give the dose as soon as possible but do not give a double dose. Make sure the dose given does not exceed the total recommended dose in any 24-hour period.

My cat is unwell, should I still give the drug?

If your cat is unwell, contact your veterinarian for advice.

What should I do if this medication is accidentally given to another pet or taken by a human?

Methimazole should only be given to the cat that it has been prescribed for:

  • Do not give to other pets in the household.
  • Do not use in cats with primary liver disease, diabetes or autoimmune disease.
  • Do not use in cats with white blood cell disorders or platelet disorders such as thrombocytopenia.
  • Do not use in pregnant or lactating cats.
  • If accidentally given to other pets, contact your veterinarian for advice.
If accidentally consumed by a human, speak to your primary care physician or poison control center. ⚠️As thiamazole is a suspected human teratogen, pregnant women and women of child-bearing age should consider having another person administer methimazole tablets or wear gloves when handling tablets. ⚠️Pregnant women and women of child-bearing age should not administer the transdermal gel form of methimazole or handle the litter containing urine from methimazole-treated cats.

Will my cat need to be monitored while on methimazole?

Hematology, biochemistry and serum total thyroxine levels (T4) should be assessed before initiating treatment and tested after 3 weeks, 6 weeks, 10 weeks, 20 weeks and thereafter every 3 months.

Further information

If you have any concerns, further information on methimazole can be found on the package insert and the following websites:

Europe & UK

USA

ℹ️Always follow the recommended dosage and contact your veterinary team or pharmacist if you have any questions regarding your cat’s medication.

Related Client Factsheets

Methimazole

Hyperthyroidism - disease and treatment

Hyperthyroidism

Hyperthyroidism: pathology

Hyperthyroidism: radio-iodine treatment

Hyperthyroidism: whole cat

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