Methimazole — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Cat	PO	Initial: 1.25-2.5 mg/cat q12h; Maintenance: 2.5-5 mg/cat q12h (up to 10 mg/cat q12h if needed)	q12h (every 12 hours)	Duration: Long-term, usually lifelong Notes: Dose should be titrated based on serum T4 levels and clinical response. Recheck T4 in 2-3 weeks after dose change. Administer with food to reduce GI upset.
Dog	PO	5-15 mg/dog q8h (for thyroid carcinoma)	q8h (every 8 hours)	Duration: Long-term, as needed Notes: Dogs are more sensitive to adverse effects; monitor liver enzymes and CBC. Use with caution.

Clinical Indications & Species Uses

General Indications

Treatment of hyperthyroidism

Dog (Canine)

Hyperthyroidism (rare, usually due to thyroid carcinoma)
Thyroid hormone suppression test (diagnostic)

Cat (Feline)

Hyperthyroidism (most common indication)
Preparation for thyroidectomy or radioactive iodine therapy

Pharmacology & Mechanism of Action

Drug Class: Antithyroid Agent | Pharmacological Group: Thioamide

Mechanism of Action: Methimazole inhibits the enzyme thyroid peroxidase, thereby blocking the iodination of tyrosine residues in thyroglobulin and the coupling of iodotyrosines to form thyroxine (T4) and triiodothyronine (T3). It does not affect existing thyroid hormone stores or circulating hormones, so clinical effects are delayed until stores are depleted. It also has mild immunosuppressive properties.

Pharmacodynamics: Methimazole reduces thyroid hormone synthesis, leading to a gradual decrease in serum T4 and T3 concentrations. The onset of action is typically 1-2 weeks, with maximal effect in 3-4 weeks. It does not affect the release of preformed hormones, so it is not effective in thyroid storm as a sole agent. In cats, it normalizes hyperthyroid state, improving clinical signs such as weight loss, polyphagia, and tachycardia.

⚡ Pharmacokinetics Summary

Absorption: Rapidly and completely absorbed from the gastrointestinal tract after oral administration. Food may slightly decrease the rate but not the extent of absorption.

Distribution: Widely distributed throughout the body, including into milk and across the placenta. It concentrates in the thyroid gland. Volume of distribution is approximately 0.6 L/kg in humans.

Metabolism: Metabolized in the liver primarily by oxidation and glucuronidation. The metabolites are inactive.

Excretion: Excreted primarily in the urine, with a small amount in feces. In cats, the elimination half-life is approximately 4-6 hours, but the duration of action is longer due to accumulation in the thyroid.

Half-Life: Cats: 4-6 hours; Dogs: 4-8 hours; Humans: 5-6 hours.

Bioavailability: Oral bioavailability is approximately 70-80% in cats and dogs.

Protein Binding: Low protein binding, approximately 0-10%.

Available Formulations & Strengths

Oral Tablet 2.5 mg, 5 mg, 10 mg (PO)

Oral Solution 5 mg/mL (PO)

Transdermal Gel 5 mg/0.1 mL (compounded) (Topical (applied to inner pinna))

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to methimazole or other thioamides
Pregnancy (especially first trimester) and lactation (unless benefits outweigh risks)
Severe hepatic disease
Agranulocytosis or severe bone marrow suppression
Use in animals with concurrent autoimmune disease (relative contraindication)

Warnings & Clinical Precautions:

Monitor for signs of hepatotoxicity, blood dyscrasias, and skin reactions.
Use with caution in animals with renal disease; may unmask renal insufficiency by lowering blood pressure.
In cats, monitor renal parameters before and during therapy.
May cause a metallic taste or anorexia; administer with food.
Transdermal gel should be applied with gloves and to a clean, hairless area; alternate ears.
Avoid handling crushed tablets or solution if pregnant or trying to conceive.
Discontinue if severe adverse effects occur (e.g., vomiting, lethargy, icterus, bleeding).

Adverse Effects & Reactions

Common:

Vomiting
Anorexia
Lethargy
Diarrhea
Facial pruritus (especially in cats)
Excoriation of face and neck

Serious / Severe:

Hepatotoxicity
Agranulocytosis
Thrombocytopenia
Hemolytic anemia
Bleeding diathesis
Immune-mediated myasthenia gravis
Lupus-like syndrome

Rare:

Cutaneous drug eruption
Arthritis
Nephrotic syndrome
Hypoglycemia
Insulin autoimmune syndrome

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Beta-blockers (e.g., propranolol)	May reduce metabolism of beta-blockers; additive bradycardia.	Moderate
Digoxin	Hyperthyroidism increases digoxin clearance; after euthyroidism, digoxin levels may increase, requiring dose reduction.	Moderate
Warfarin	Methimazole may enhance anticoagulant effect by reducing vitamin K-dependent clotting factor synthesis.	High
Theophylline	Clearance of theophylline may be reduced, increasing risk of toxicity.	Moderate
Iodine-containing compounds	May reduce the efficacy of methimazole; avoid concurrent use.	Moderate

Overdose & Toxicity Management

Signs of Toxicity:

Vomiting
Diarrhea
Anorexia
Lethargy
Hepatotoxicity
Bone marrow suppression
Hypothyroidism
Skin reactions

Emergency Treatment Protocol: Induce emesis if recent ingestion and animal is stable. Administer activated charcoal. Provide supportive care including IV fluids, antiemetics, and monitoring of liver function and CBC. For severe bone marrow suppression, consider colony-stimulating factors. Thyroid hormone replacement may be needed if hypothyroidism develops.

Food Animal Withdrawal Times

Not approved for use in food animals. Do not use in animals intended for food.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature 20-25°C (68-77°F), excursions permitted between 15-30°C (59-86°F).

Light Sensitivity: Light-sensitive — protect from direct exposure.

Handling & Special Conditions: Protect from light and moisture. Keep container tightly closed. Transdermal gel should be stored at room temperature and protected from heat.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Approved for use in cats (Felimazole). Not approved for dogs or other species, but may be used extra-label.

Extra-Label (Off-Label) Use: In the US, methimazole is FDA-approved for use in cats (Felimazole). Extra-label use in other species is permitted under AMDUCA with appropriate veterinary oversight and withdrawal times for food animals must be established.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Methimazole is the drug of choice for long-term management of feline hyperthyroidism. It is preferred over propylthiouracil due to fewer adverse effects. Therapy should be initiated at a low dose and titrated based on serum T4 levels. Recheck T4 and clinical signs 2-3 weeks after initiation or dose change. Once stable, monitor every 3-6 months. Renal function should be assessed before and during treatment, as hyperthyroidism can mask underlying chronic kidney disease. If azotemia develops, consider alternative therapies such as radioactive iodine or thyroidectomy. Transdermal gel is an option for cats that are difficult to pill, but absorption may be variable. Adverse effects are usually reversible upon discontinuation. In dogs, methimazole is used for thyroid carcinoma, but it is less well tolerated; monitor closely for hepatotoxicity and blood dyscrasias.

References & Literature

📚 Plumb's Veterinary Drug Handbook
📚 Papich Veterinary Pharmacology and Therapeutics
📚 Compendium of Veterinary Products (CVP)