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Veterinary Medication🐈 Catsendocrine

Methimazole (Felimazole)

🗣️ Pronounced:meth-IM-uh-zohl
In One Sentence (Plain English)

An antithyroid drug that inhibits the production of thyroid hormones to manage feline hyperthyroidism.

Editorial Status: Verified from peer-reviewed veterinary sources (v1.0)
Updated: 2026-01-15

💬 In Plain English

Methimazole is the primary daily medication used to manage hyperthyroidism in older cats. It does not cure or destroy the underlying thyroid tumor, but it blocks the thyroid gland from incorporating iodine into new thyroid hormones. Within 2 to 4 weeks of starting treatment, circulating thyroid hormone levels drop back into the normal range. The cat’s ravenous appetite settles down, weight stabilizes, and rapid heart rates normalize. It is available as oral tablets, flavored liquids, or a transdermal gel applied to the inner ear flap.

🔬 Clinical & Technical Depth

For Vets & Technicians

Formal Clinical Definition

Thionamide antithyroid compound that blocks thyroid peroxidase (TPO), preventing the iodination of tyrosine residues on thyroglobulin and coupling of iodotyrosines, thereby inhibiting synthesis of T3 and T4.

Why Veterinarians Use It

Veterinarians prescribe methimazole as lifelong medical control for feline hyperthyroidism or as a diagnostic trial before curative radioactive iodine (I-131) therapy to assess renal tolerance.

Clinical Context Encountered

Prescribed routinely for cats over 10 years of age diagnosed with elevated Total T4 and clinical hyperthyroidism.

Clinical Reference Guide (Veterinary Medication)

Drug ClassThionamide Antithyroid Agent
Common Brands: Felimazole, Tapazole, Compounded transdermal lipoderm gel

What Owners Must Know

  • Wear gloves when applying transdermal gel to the cat’s ear, and rotate ears each dose (clean residual wax before applying new dose)
  • Must be administered consistently every day; stopping medication causes thyroid hormones to surge within 48 to 72 hours
  • Wash hands thoroughly after handling pills or medicated cats

Potential Side Effects

  • Vomiting, anorexia, and lethargy (most common, often transient during initial weeks)
  • Severe facial pruritus and self-mutilation excoriations (requires permanent drug discontinuation)
  • Hepatopathy / elevated liver enzymes
  • Bone marrow suppression (agranulocytosis, severe leukopenia, or thrombocytopenia in <2%)
  • Unmasking of underlying chronic kidney disease as glomerular filtration rate normalizes
🚨 Red Flag Warnings (When to Contact an Emergency Vet)
  • •Facial pruritus (violent scratching at the face, muzzle, and neck resulting in bleeding excoriations) is an idiosyncratic hypersensitivity reaction that requires immediate discontinuation of methimazole.
  • •Always wear non-permeable gloves when applying transdermal methimazole to a cat’s ear flap to prevent absorbing the medication through your own skin.

❓ Questions to Ask Your Veterinarian

Take these actionable questions with you to your next clinic visit or appointment:

  • 1Should we start with oral tablets or transdermal inner-ear pen gel (useful for difficult-to-pill cats)?
  • 2When should we recheck Total T4, CBC, and kidney values (typically 3 to 4 weeks after starting or adjusting dose)?
  • 3Has my cat’s kidney function (creatinine, SDMA, USG) been evaluated to ensure lowering thyroid hormone does not unmask kidney failure?
  • 4Is my cat a good candidate for permanent, curative radioactive iodine (I-131) therapy?

Academic & Clinical References