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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
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 →

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
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 →

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.
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 →

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.
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 →

References

References

The PetCare.AI drug reference is built from 13 authoritative sources cited across 580 drug monographs.

Textbooks & handbooks — 5

  • Plumb's Veterinary Drug Handbook
  • Withrow & MacEwen's Small Animal Clinical Oncology
  • Merck Veterinary Manual
  • NRC Nutrient Requirements of Dogs and Cats
  • Veterinary Pharmacology and Therapeutics (Riviere & Papich)

Clinical guidelines & consensus — 4

  • ASPCA Animal Poison Control Center Guidelines
  • AAHA Diabetes Management Guidelines
  • ASPCA Poison Control Guidelines
  • RECOVER CPR Guidelines

Journals & peer-reviewed studies — 2

  • EPIC Study (J Vet Intern Med 2016)
  • JVIM FAT CAT Study

Regulatory & approvals — 1

  • CDSCO Veterinary Drug Approval Registry (1969–2026)

Databases — 1

  • Washington State University VCPL MDR1 Database
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