Methimazole
1-methyl-1,3-dihydro-2H-imidazole-2-thione (Thiamazole)
Also known as: Tapazole · Felimazole · Northyx · Antitiroide · Danantizol · Favistan · Mercaptizol · Metibasol · Strumazol · Thacapzol · Thycapzol · Thyrozol · Tirodril · Unimazole · Thiafeline · Thyronorm · Thiamazole · Mercazolylum · Methylmercaptoimidazole · Thiamazolum · Tiamazol · Methimazole · Mercazolil
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Dogs
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| As an investigative method to reduce nephrotoxicity associated with cisplatin therapy | 40 mg/kg IV over one minute prior to cisplatin. | IV | once | Prior to cisplatin | 🌎 NA |
| Hyperthyroidism | 2.5-5 mg/dog | PO | q12h | Long-term | 🌍 EU |
- As an investigative method to reduce nephrotoxicity associated with cisplatin therapy: No commercially available parenteral product in USA at time of writing.
- Hyperthyroidism: Dose depends on size.
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Hyperthyroidism | The starting dose is 2.5 mg administered every 12 hours. Following 3 weeks of treatment, the dose should be titrated to effect based on individual serum total T4 (TT4) levels and clinical response. Dose adjustments should be made in 2.5 mg increments. The maximum total dosage is 20 mg per day divided, not to exceed 10 mg as a single administration. | PO | q12h | Lifelong | 🌎 NA |
| Hyperthyroidism (cats with azotemia or for clients declining radioiodine) | 1.25-5 mg per cat twice daily (start at lower end. | PO | q12h | Lifelong | 🌎 NA |
| Hyperthyroidism (transdermal administration) | 2.5 mg to inner pinna q12h. | topical | q12h | Lifelong | 🌎 NA |
| Hyperthyroidism (gradual titration) | Initially, 2.5 mg (total dose) PO once a day for 2 weeks. If adverse reactions not noted... increase to 2.5 mg PO twice daily and the same parameters are checked in another 2 weeks. The dosage should then be increased every 2 weeks by 2.5 mg per day until serum T4 is between 13 and 26 nmol/L or adverse effects develop. | PO | q24h initially, then q12h | Lifelong | 🌎 NA |
| Hyperthyroidism (based on renal status) | If no signs of renal insufficiency/failure, begin at 5 mg (total dose) PO twice daily in cases with severely increased T4 levels. If renal insufficiency present (or not sure), start at 2.5 mg twice daily. If azotemia and overt renal failure, start at 1.25 mg twice a day. | PO | q12h | Lifelong | 🌎 NA |
| Hyperthyroidism | 2.5 mg/cat | PO | q12h | Long-term | 🌍 EU |
| Hyperthyroidism | Apply transdermal gel to pinna | Topical | q12h | Long-term | 🌍 EU |
- Hyperthyroidism (transdermal administration): Methimazole (50 mg/mL; 5 mg/0.1 mL) in PLO. Person applying should wear gloves or finger cots. Somewhat lower efficacy than PO (67% vs 82% euthyroid at 4 weeks). Lower incidence of GI effects.
- Hyperthyroidism (based on renal status): Monitor in 1-2 weeks (T4, CBC with platelet count, renal blood parameters, urinalysis).
- Hyperthyroidism: 2 to 3 weeks of treatment generally needed to establish euthyroidism. Adjust dose based on T4 monitoring.
- Hyperthyroidism: Used particularly in fractious cats or those with GI side effects. Wear gloves.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Methimazole is a thioimidazole-derivative antithyroid drug and is considered the gold standard medical treatment for feline hyperthyroidism in North America.
Key clinical points:
- Primary Use: Management of hyperthyroidism in cats. It effectively lowers circulating thyroid hormone levels, resolving clinical signs like weight loss, polyphagia, and tachycardia.
- Alternative Routes: For cats that are difficult to pill, methimazole can be compounded into a transdermal PLO gel applied to the inner pinna, which has shown good clinical efficacy (though slightly lower than oral) with potentially fewer gastrointestinal side effects.
- Investigational Use: In dogs, it has been used prophylactically as an intravenous injection to prevent cisplatin-induced nephrotoxicity.
- Clinical Pearl: Methimazole manages but does not cure hyperthyroidism. Lifelong daily therapy is required unless definitive treatment (radioactive iodine or thyroidectomy) is pursued.
Mechanism of action
Methimazole acts directly within the thyroid gland to block the synthesis of thyroid hormones (T3 and T4).
- Mechanism: It actively inhibits the enzyme thyroid peroxidase (TPO).
- Pathway: Inhibition of TPO → prevents the oxidation of iodide to iodine → blocks the incorporation of iodine into tyrosyl residues of thyroglobulin → inhibits the coupling of iodinated tyrosyl residues to form iodothyronine (T3 and T4).
- Important Note: Methimazole has no effect on the release or activity of thyroid hormones that are already formed and stored, or those currently in systemic circulation. This accounts for the 1 to 3 week lag time often seen before serum T4 levels significantly decrease and clinical improvement is noted.
Safety & warnings
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
- Animals with known hypersensitivity to thiamazole or carbimazole
Adverse effects
- Vomiting
- Depression/Lethargy
- Transient eosinophilia, leukopenia, and lymphocytosis
- Self-induced facial excoriations (pruritus)
- Bleeding/Coagulopathies
- Agranulocytosis
- Positive direct antiglobulin test (DAT)
- Positive ANA (in chronic use >6 months)
- Acquired myasthenia gravis (rare)
- Inappetence / Anorexia
- Jaundice / Hepatopathy
- Cytopenias (thrombocytopenia, leukopenia, anemia)
- Immune-mediated diseases (e.g., myasthenia gravis)
- Dermatological changes (pruritus, facial excoriation, alopecia)
- Decreased GFR (unmasking of renal failure)
Precautions
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
Methimazole can reduce hepatic oxidation of benzimidazoles and increase blood levels.
A reduction in dose may be needed when the patient becomes euthyroid.
Potential for increased risk for hepatotoxicity; increased monitoring (LFT's) necessary.
Methimazole may decrease digoxin efficacy, but a reduction in dose may be needed when the patient becomes euthyroid.
Concurrent use of phenobarbital may reduce the clinical effectiveness of methimazole.
A reduction in dose may be needed when the patient becomes euthyroid.
Anticoagulants may be potentiated by methimazole. Increased monitoring of anticoagulant effect is warranted.
Reduce hepatic oxidation and may lead to increased circulating thiamazole concentrations
Thiamazole interferes with radioiodine uptake; must be discontinued prior to I-131 treatment
Concurrent use is not recommended as it confounds dietary management and iodine restriction
Monitoring
- Baseline and every 2-3 weeks for the first 3 months: CBC, platelet count, Serum T4
- Liver function tests and ANA if indicated by symptomatology
- After stabilization (at least 3 months): T4 at 3-6 month intervals
- Renal parameters (BUN, Creatinine, USG) to monitor for unmasked kidney disease
- Cats receiving >10 mg/day require more frequent monitoring
- Serum total T4 (measure 4-6 hours post-pill)
- Haematology (CBC for cytopenias)
- Biochemistry (BUN, Creatinine, Liver enzymes)
- Monitor at 3, 6, 10, and 20 weeks after starting therapy, and every 3 months thereafter
Pharmacokinetics
Half-life
Absorption
In normal cats, oral bioavailability is highly variable (45-98%). There is usually a 1-3 week lag time between starting the drug and significant reductions in serum T4.
Distribution
Volume of distribution in cats is 0.12-0.84 L/kg. Methimazole actively concentrates in thyroid tissue. High levels cross the placenta and are detected in breast milk.
Metabolism
Undergoes hepatic metabolism.
Elimination
Eliminated primarily via the kidneys and feces after hepatic metabolism.
Overdose
Acute toxicity from overdosage mirrors severe adverse effects: agranulocytosis, hepatopathy, and thrombocytopenia are the most serious concerns.
Treatment:
- Follow standard protocols for oral ingestion (empty stomach if not contraindicated, administer activated charcoal).
- Provide symptomatic and supportive care (e.g., IV fluids, hepatoprotectants, broad-spectrum antibiotics if neutropenic).
Available products
Formulations
- Oral tablets (sugar-coated, plain, scored)
- Transdermal gel (compounded)
- Intravenous injection (investigational/compounded)
- Oral: 1.25 mg tablet
- Oral: 2.5 mg tablet
- Oral: 5 mg tablet
- Oral: 5 mg/ml solution
- Transdermal gel (named patient basis/compounded)
Veterinary
- Methimazole Tablets (sugar-coated): 2.5 mg & 5 mg (Felimazole - Dechra)
- Felimazole 1.25 mg, 2.5 mg, 5 mg tablets
- Thiafeline 2.5 mg, 5 mg tablets
- Thyronorm 5 mg/ml oral solution
Human-labeled
- Methimazole Tablets (plain & scored): 5 mg, 10 mg, 15 mg & 20 mg (Tapazole, Northyx, generic)
Regulatory status
POM-V classification.
POM-V classification.
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Store tablets in well-closed, light-resistant containers at room temperature; protect from moisture. Compounded transdermal PLO gels typically have a guaranteed stability of 2 weeks.
Client information
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
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.