Methimazole
Also known as: Tapazole, Felimazole, Northyx, Antitiroide, Danantizol, Favistan, Mercaptizol, Metibasol, Strumazol, Thacapzol, Thycapzol, Thyrozol, Tirodril, Unimazole, Thiafeline, Thyronorm
### Understanding Your Pet's Medication **Methimazole** is a daily medication used to manage hyperthyroidism (an overactive thyroid gland) in cats. - **How it works**: It blocks the production of excess thyroid hormones, helping your cat regain normal weight, heart rate, and behavior. - **Not a cure**: This medication *controls* the disease but does not cure it. Your cat will likely need this medication for life. - **Patience is key**: It may take 1 to 3 weeks to see noticeable improvement in your cat's symptoms. ### Administration Tips - The pills have a very bitter taste. Do not crush them unless instructed, as your cat may refuse to eat the food it is mixed with. - If you are struggling to pill your cat, ask your veterinarian about a **transdermal gel** that can be rubbed onto the hairless part of your cat's ear. ### Safety Warnings > **Human Safety**: Methimazole can affect human thyroid function and fetal development. **Pregnant women, women trying to conceive, and nursing mothers should wear gloves** when handling the medication, the cat's litter box, or bodily fluids. ### What to Watch For Contact your veterinarian immediately if your cat experiences: - Vomiting or loss of appetite (especially in the first few weeks) - Extreme lethargy or weakness - Intense scratching at the face or neck - Unexplained bruising or bleeding
Dosage must be set by your veterinarian for your specific pet. Never give human medication or another pet’s prescription without veterinary guidance.
What it is used for
- Hyperthyroidism
- Hyperthyroidism (cats with azotemia or for clients declining radioiodine)
- Hyperthyroidism (transdermal administration)
- Hyperthyroidism (gradual titration)
- Hyperthyroidism (based on renal status)
- Hyperthyroidism
- Hyperthyroidism
- As an investigative method to reduce nephrotoxicity associated with cisplatin therapy
- Hyperthyroidism
How it is given
Possible side effects
- Vomiting
- Anorexia
- Depression/Lethargy
- Transient eosinophilia, leukopenia, and lymphocytosis
- Self-induced facial excoriations (pruritus)
- Bleeding/Coagulopathies
- Hepatopathy
- Thrombocytopenia
- Agranulocytosis
- Positive direct antiglobulin test (DAT)
- Positive ANA (in chronic use >6 months)
- Acquired myasthenia gravis (rare)
- Inappetence / Anorexia
- Jaundice / Hepatopathy
Cautions & contraindications
- Hypersensitivity to methimazole, carbimazole, or polyethylene glycol
- Autoimmune disease
- Primary liver disease
- Renal failure
- Hematologic disorders or coagulopathies
- Pregnant or lactating queens
- Pregnant queens
- Lactating queens
- Animals with known hypersensitivity to thiamazole or carbimazole
- > **Black Box / Handling Warning**: Pregnant women, women who may become pregnant, and nursing mothers should wear gloves when handling tablets, litter, or bodily fluids of treated cats. - **Renal Unmasking**: Restoring euthyroidism can decrease glomerular filtration rate (GFR) and unmask underlying chronic kidney disease in cats. Careful monitoring of renal values is essential. - **Hematologic & Hepatic Monitoring**: Severe adverse effects like agranulocytosis and hepatotoxicity usually occur within the first 3 months. Routine bloodwork monitoring is critical during this phase. - **Nursing**: Kittens should be placed on a milk replacer if the mother is receiving the drug, as it is excreted in milk and can cause fetal/neonatal hypothyroidism.
Drug interactions
- Benzimidazole antiparasitics: Methimazole can reduce hepatic oxidation of benzimidazoles and increase blood levels.
- Beta-blockers: A reduction in dose may be needed when the patient becomes euthyroid.
- Bupropion: Potential for increased risk for hepatotoxicity; increased monitoring (LFT's) necessary.
- Digoxin: Methimazole may decrease digoxin efficacy, but a reduction in dose may be needed when the patient becomes euthyroid.
- Phenobarbital: Concurrent use of phenobarbital may reduce the clinical effectiveness of methimazole.
- Theophylline: A reduction in dose may be needed when the patient becomes euthyroid.
- Warfarin: Anticoagulants may be potentiated by methimazole. Increased monitoring of anticoagulant effect is warranted.
- Benzimidazoles: Reduce hepatic oxidation and may lead to increased circulating thiamazole concentrations
- Iodine-131 (Radioiodine): Thiamazole interferes with radioiodine uptake; must be discontinued prior to I-131 treatment
- Low-iodine prescription diets: Concurrent use is not recommended as it confounds dietary management and iodine restriction
Frequently asked questions
What is Methimazole used for in pets?
### Understanding Your Pet's Medication **Methimazole** is a daily medication used to manage hyperthyroidism (an overactive thyroid gland) in cats. - **How it works**: It blocks the production of excess thyroid hormones, helping your cat regain normal weight, heart rate, and behavior. - **Not a cure**: This medication *controls* the disease but does not cure it. Your cat will likely need this medication for life. - **Patience is key**: It may take 1 to 3 weeks to see noticeable improvement in your cat's symptoms. ### Administration Tips - The pills have a very bitter taste. Do not crush them unless instructed, as your cat may refuse to eat the food it is mixed with. - If you are struggling to pill your cat, ask your veterinarian about a **transdermal gel** that can be rubbed onto the hairless part of your cat's ear. ### Safety Warnings > **Human Safety**: Methimazole can affect human thyroid function and fetal development. **Pregnant women, women trying to conceive, and nursing mothers should wear gloves** when handling the medication, the cat's litter box, or bodily fluids. ### What to Watch For Contact your veterinarian immediately if your cat experiences: - Vomiting or loss of appetite (especially in the first few weeks) - Extreme lethargy or weakness - Intense scratching at the face or neck - Unexplained bruising or bleeding
Is Methimazole safe for my pet?
Dosage must be set by your veterinarian for your specific pet. Never give human medication or another pet’s prescription without veterinary guidance.
This article is for general education and is not a substitute for professional veterinary advice. If your pet is unwell, please consult a veterinarian.
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