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Methimazole

Prescription
Thioureylene antithyroid agent
Last reviewed 19 Apr 2026 · PetCare.AI Editorial Team
Species
Cat, Dog
Brands
4 available
Interactions
4 documented
Formulations
4

Mechanism of action

Inhibits thyroid peroxidase enzyme, blocking iodination of tyrosine and coupling of iodotyrosines, thus reducing thyroid hormone (T3/T4) synthesis

At a glance

Class
Thioureylene antithyroid agent
Schedule
Prescription
Storage
Store below 25°C, protect from light. Wear gloves when handling — teratogenic to humans.

Dosing

🐈

Cat

Hyperthyroidism
Dose
1.25–5 mg/cat
Route
PO
Frequency
BID
Max dose
10 mg/dose; 20 mg/day
Duration: Lifelong (unless radioiodine or surgery); start low and titrate based on T4
Hyperthyroidism (transdermal)
Dose
2.5–5 mg/cat
Route
Transdermal (ear pinna)
Frequency
BID
Max dose
10 mg/dose; 20 mg/day
Duration: Lifelong; may need higher dose than oral
🐕

Dog

Hyperthyroidism (rare — thyroid carcinoma)
Dose
5–15 mg/dog
Route
PO
Frequency
TID
Max dose
20 mg/dose; 60 mg/day
Duration: Pre-surgical stabilisation or palliative

Formulations

💊

Tablet — 2

Strength
2.5mg
Available in India
Strength
5mg
Available in India
💊

Tablet (coated — vet formulation) — 1

Strength
2.5mg
💊

Transdermal gel — 1

Strength
50mg/mL (compounded)

Storage

Store below 25°C, protect from light. Wear gloves when handling — teratogenic to humans.

Safety

Absolute contraindications — do not use

  • Known hypersensitivity to thioureylenes
    Immune-mediated adverse effects are dose-related but can recur on rechallenge
  • Autoimmune disease
    May exacerbate autoimmune conditions

Use with caution

  • Pre-existing liver disease
    Hepatotoxicity is a known adverse effect — monitor closely
  • Pre-existing renal disease (cats)
    Treating hyperthyroidism unmasks CKD by reducing GFR — trial methimazole for 2-4 weeks and recheck renal values before committing to permanent treatment

Adverse effects

Common
Vomiting
Anorexia
Lethargy
Facial excoriation/pruritus
Serious
Hepatotoxicity
Blood dyscrasias (neutropenia, thrombocytopenia, agranulocytosis)
Myasthenia gravis-like syndrome
Haemolytic anaemia

Monitoring parameters

Total T4 (2-3 weeks post-start, then every 3-6 months)CBC (2-3 weeks, then every 3 months initially)Liver enzymesRenal function (BUN, Creatinine — critical to unmask CKD)Body weight

Interactions

Contraindicated — 1

Carbimazole
contraindicated
Carbimazole is a prodrug converted to methimazole. Combining them is effectively double-dosing the same drug, causing severe hypothyroidism and myelosuppression.
Management: NEVER combine. Use one or the other. When switching, discontinue first agent before starting the second.

Moderate — 2

Phenobarbital
moderate
Phenobarbital induces hepatic metabolism, potentially altering methimazole clearance. Also, phenobarbital lowers total T4 (diagnostic confusion).
Management: Monitor thyroid levels more frequently. Adjust methimazole dose based on T4 response.
Benazepril
moderate
Methimazole treatment unmasks pre-existing CKD (previously masked by hyperthyroidism-induced hyperfiltration). Adding ACE inhibitor further reduces GFR.
Management: Monitor BUN/creatinine at 2 and 4 weeks after starting methimazole. Adjust methimazole dose to maintain mild hyperthyroidism if CKD is severe.

Minor — 1

Enalapril
minor
No direct interaction; however both can affect renal perfusion. Commonly co-prescribed in hyperthyroid cats with concurrent renal disease.
Management: Monitor renal values closely when initiating methimazole, as correcting hyperthyroidism may unmask CKD.

Brands

International

Tapazole
Lilly
Felimazole
Dechra

India

Thyrozole
Merck
Methimez
Sun Pharma

FAQs

Frequently asked questions

›What is Methimazole?
Methimazole is a thioureylene antithyroid agent used in pets. Inhibits thyroid peroxidase enzyme, blocking iodination of tyrosine and coupling of iodotyrosines, thus reducing thyroid hormone (T3/T4) synthesis
›What is Methimazole used for in pets?
Methimazole is used in veterinary medicine for: Hyperthyroidism; Hyperthyroidism (transdermal); Hyperthyroidism (rare — thyroid carcinoma).
›What is the Methimazole dose for cats?
For cats, Methimazole is typically dosed as follows — Hyperthyroidism: 1.25–5 mg/cat PO BID; Hyperthyroidism (transdermal): 2.5–5 mg/cat Transdermal (ear pinna) BID. Always consult your veterinarian for a dose tailored to your pet's weight, age, and condition.
›What is the Methimazole dose for dogs?
For dogs, Methimazole is typically dosed as follows — Hyperthyroidism (rare — thyroid carcinoma): 5–15 mg/dog PO TID. Always consult your veterinarian for a dose tailored to your pet's weight, age, and condition.
›What are the side effects of Methimazole?
Common: Vomiting, Anorexia, Lethargy, Facial excoriation/pruritus. Serious (call your vet immediately): Hepatotoxicity, Blood dyscrasias (neutropenia, thrombocytopenia, agranulocytosis), Myasthenia gravis-like syndrome, Haemolytic anaemia.
›Does Methimazole need a prescription?
Yes. Methimazole is a prescription medication and should only be administered under veterinary supervision.
›When should Methimazole not be used?
Do not use Methimazole if: Known hypersensitivity to thioureylenes; Autoimmune disease.

References

References

Textbooks & handbooks

  • Plumb, D.C. Plumb's Veterinary Drug Handbook. 10th ed., Wiley-Blackwell, 2023.
  • Vail, D.M., Thamm, D.H., & Liptak, J.M. (eds.). Withrow & MacEwen's Small Animal Clinical Oncology. 6th ed., Saunders/Elsevier, 2020.
  • Riviere, J.E., & Papich, M.G. (eds.). Veterinary Pharmacology and Therapeutics. 10th ed., Wiley-Blackwell, 2018.
  • National Research Council. Nutrient Requirements of Dogs and Cats. National Academies Press, Washington DC, 2006.
  • The Merck Veterinary Manual. Merck & Co., Online edition. https://www.merckvetmanual.com/

Clinical guidelines & consensus

  • Fletcher, D.J., Boller, M., Brainard, B.M., et al. "RECOVER Evidence and Knowledge Gap Analysis on Veterinary CPR." Journal of Veterinary Emergency and Critical Care, 2012;22(S1):S102–S131.
  • American Animal Hospital Association. 2018 AAHA Diabetes Management Guidelines for Dogs and Cats. AAHA Press.

Journals & peer-reviewed studies

  • Hogan, D.F., Fox, P.R., Jacob, K., et al. "Secondary prevention of cardiogenic arterial thromboembolism in the cat: The FAT CAT study." Journal of Veterinary Cardiology, 2015;17(Suppl 1):S306–S317.
  • Boswood, A., Häggström, J., Gordon, S.G., et al. "Effect of Pimobendan in Dogs with Preclinical Myxomatous Mitral Valve Disease and Cardiomegaly: The EPIC Study — A Randomized Clinical Trial." Journal of Veterinary Internal Medicine, 2016;30(6):1765–1779.
  • ASPCA Animal Poison Control Center. Toxicology and Poison Management Guidelines. American Society for the Prevention of Cruelty to Animals. https://www.aspca.org/pet-care/animal-poison-control

Regulatory & approvals

  • Central Drugs Standard Control Organisation (CDSCO), Government of India. Veterinary Drug Approval Registry, 1969–2026. Directorate General of Health Services. https://cdsco.gov.in/

Databases

  • Washington State University, College of Veterinary Medicine. Veterinary Clinical Pharmacology Laboratory (VCPL) — MDR1 Multidrug Sensitivity Database. https://vcpl.vetmed.wsu.edu/
Educational reference only
This information is provided for educational purposes and is not a substitute for professional veterinary advice, diagnosis, or treatment. Always consult a qualified veterinarian before administering any medication to your pet. Find a vet near you →