# Methimazole: veterinary dose and monograph

> Methimazole is a veterinary drug. Antithyroid thionamide — blocks thyroid peroxidase, inhibiting thyroid-hormone synthesis. Indications include feline hyperthyroidism — long-term control; stabilisation / renal trial before radioiodine or thyroidectomy. This monograph lists 3 reference doses for cats, each with route, frequency and source.

Source: https://vetmasterji.com/drugs/methimazole · Last reviewed: 2026-10-01

## At a glance

- **Drug class:** Veterinary drug
- **Also known as:** Tapazole, Felimazole, Thiamazole
- **Species with doses:** Cats
- **General dose range:** 0.25–0.5 mg/kg PO q12h — check the species tables for the indication
- **Routes:** PO, Topical
- **Last reviewed:** 1 October 2026

## Indications

- Feline hyperthyroidism — long-term control
- Stabilisation / renal trial before radioiodine or thyroidectomy

## Methimazole dose by species

Doses are per administration unless stated. mg/kg doses scale with body weight; per-animal, per-quarter and infusion-rate doses are shown as the reference writes them.

### Dogs

*Methimazole doses for dogs*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| No reference dose for this species | Not established in Papich 5e or Plumb’s 10e — do not extrapolate from another species | — | — | — |

- No reference dose for this species: Neither Papich 5e nor Plumb’s gives a canine hyperthyroidism dose (Plumb’s lists only an investigational 40 mg/kg IV dose for cisplatin nephroprotection). Canine hyperthyroidism is rare (usually thyroid carcinoma) — manage with a specialist protocol.

### Cats

*Methimazole doses for cats*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Hyperthyroidism (oral) | 1.25–2.5 mg/cat | PO | q12h, titrate by T4 | — |
| Hyperthyroidism (transdermal, twice daily) | 2.5 mg/cat | Topical | q12h to the inner pinna | — |
| Hyperthyroidism (transdermal, once daily) | 5–10 mg/cat | Topical | q24h to the inner pinna | — |

- Hyperthyroidism (oral): Papich 5e: 2.5 mg/cat q12h PO for 7–14 days, then up to 5–10 mg/cat q12h if needed; some cats manage once daily. Plumb’s p.801 (Felimazole label): start 2.5 mg q12h, adjust in 2.5 mg steps after 3 weeks; maximum 20 mg/day, no more than 10 mg per dose. Azotaemic cats start at 1.25 mg q12h (Ward 2003; Trepanier 2007). Monitor T4, CBC/platelets, liver and renal values.
- Hyperthyroidism (transdermal, twice daily): Plumb’s p.801 (Trepanier 2006): 2.5 mg in PLO gel (5 mg/0.1 mL) to the inner pinna q12h — somewhat less effective than oral (67% vs 82% euthyroid at 4 weeks) but fewer GI effects. Wear gloves; alternate ears.
- Hyperthyroidism (transdermal, once daily): Papich 5e: 10 mg per cat transdermally once daily, adjusted by T4; some cats need twice-daily dosing and some are maintained on 5 mg once daily. Wear gloves; alternate ears.

## Contraindications

Pre-existing hepatopathy or significant cytopenias. Caution in azotaemia — restoring euthyroidism can unmask reduced GFR. Handle with gloves (transdermal gel / crushed tablets) — teratogenic.

## Species-specific warnings

> **Caution:** Cats: Watch for hepatotoxicity and neutropenia / thrombocytopenia in the first 3 months — recheck CBC and stop if they occur.

## Adverse effects

- GI upset, anorexia, vomiting (early, often transient)
- Facial / cervical excoriation from self-trauma (pruritus)
- Hepatotoxicity; blood dyscrasias (neutropenia, thrombocytopenia)
- Unmasking of azotaemia as euthyroidism is restored

## Drug interactions

- Benzimidazole antiparasitics (fenbendazole, albendazole and relatives): methimazole reduces their hepatic oxidation and INCREASES blood levels — relevant when deworming a hyperthyroid cat
- Beta-blockers: the veterinary label advises a dose reduction may be needed once the patient becomes euthyroid (hyperthyroidism was driving the need for them)
- Theophylline: same principle — dose reduction may be needed on becoming euthyroid
- Digoxin: methimazole may decrease digoxin efficacy; the veterinary label likewise advises dose reduction once euthyroid
- Phenobarbital: concurrent use may REDUCE methimazole clinical effectiveness
- Bupropion: potential for increased hepatotoxicity risk — monitor liver enzymes
- Warfarin: complex. Hyperthyroidism increases metabolism of vitamin K clotting factors and so increases anticoagulant sensitivity; by reversing the hyperthyroid state methimazole may reduce warfarin effect. But a patient euthyroid on methimazole and receiving warfarin can develop hypoprothrombinaemia if methimazole is STOPPED and they become thyrotoxic again. The veterinary label states anticoagulants may be potentiated — either way, increase monitoring of anticoagulant effect

## Pregnancy and lactation

Teratogenic — avoid in pregnancy; owners should wear gloves when handling tablets or the transdermal gel.

## Monitoring

- Total T4 at 2–3 weeks, then periodically — aim for low-normal
- CBC and platelets (dyscrasias usually arise in the first 3 months)
- Renal values (BUN / creatinine) — euthyroidism can reveal CKD
- Liver enzymes if anorexia / icterus develops

## Toxicity and overdose

Plumb's p.801: acute toxicity in overdose mirrors the adverse-effect profile, with AGRANULOCYTOSIS, HEPATOPATHY and THROMBOCYTOPENIA the most serious effects. Treatment is standard oral-ingestion management — empty the stomach if not contraindicated, give activated charcoal — plus symptomatic and supportive care. In practice the dangerous reactions are idiosyncratic rather than dose-dependent and cluster in the first 3 months: facial excoriation, hepatotoxicity, blood dyscrasias and, less commonly, myasthenia gravis or immune-mediated haemolytic anaemia. This is why CBC and liver enzymes are checked at intervals during induction rather than only when the cat looks unwell.

## Clinical pearls

- Recheck T4 and renal values 2–3 weeks after starting — a reversible trial protects you from rendering a CKD cat euthyroid and azotaemic before definitive therapy
- Transdermal gel to the inner ear is invaluable for un-pillable cats, though onset is slower and control less tight
- Facial excoriation and cytopenias are the classic reasons to switch to radioiodine / surgery

## Mechanism of action

Plumb's p.800: thioamide antithyroid agent. Interferes with incorporation of iodine into the tyrosyl residues of thyroglobulin, inhibiting thyroid hormone SYNTHESIS, and additionally prevents iodinated tyrosyl residues coupling to form iodothyronine. The clinically decisive limitation: methimazole has NO effect on the release or activity of thyroid hormone already formed or already circulating — which is why there is a 1–3 week lag before a euthyroid state is reached, and why it does nothing for thyroid storm on its own. It concentrates in thyroid tissue and its biological effect persists beyond measurable blood levels (plasma half-life in cats is only 2.3–10.2 hours, but dosing intervals are far longer). Carbimazole is a prodrug converted almost entirely to methimazole in vivo, and must be dosed at roughly TWICE the methimazole dose for equivalent serum levels.

## Formulations and products

- Methimazole 2.5 mg tablet (Felimazole)
- Methimazole 5 mg tablet
- Methimazole transdermal gel 5 mg/0.1 mL (PLO)

## Before you use this dose

> **Caution:** Reference doses for veterinary professionals. Confirm the dose against the product label, the patient’s condition, current guidance and local regulations before use; the attending veterinarian remains responsible for every clinical decision.

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## Frequently asked questions

### Can Methimazole be given to dogs?

Not established in Papich 5e or Plumb’s 10e — do not extrapolate from another species (No reference dose for this species).

### What is the dose of Methimazole for cats?

Reference doses for Methimazole in cats: 1.25–2.5 mg/cat PO q12h, titrate by T4 (Hyperthyroidism (oral)); 2.5 mg/cat Topical q12h to the inner pinna (Hyperthyroidism (transdermal, twice daily)); 5–10 mg/cat Topical q24h to the inner pinna (Hyperthyroidism (transdermal, once daily)). Confirm against the label and the patient before use.

### When should Methimazole not be used?

Pre-existing hepatopathy or significant cytopenias. Caution in azotaemia — restoring euthyroidism can unmask reduced GFR. Handle with gloves (transdermal gel / crushed tablets) — teratogenic.

### What are the side effects of Methimazole in animals?

GI upset, anorexia, vomiting (early, often transient); Facial / cervical excoriation from self-trauma (pruritus); Hepatotoxicity; blood dyscrasias (neutropenia, thrombocytopenia); Unmasking of azotaemia as euthyroidism is restored.

## Sources

- Plumb's Veterinary Drug Handbook
- Merck Veterinary Manual, 11th edition
- Papich, Papich Handbook of Veterinary Drugs, 5th edition

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VetMasterJi is a clinical decision-support and education platform for veterinary professionals and students. It does not provide veterinary advice, diagnosis or treatment for specific animals. All drug doses, calculators and differentials are references that must be independently verified before clinical use; the attending registered veterinarian remains solely responsible for every clinical decision.