Carbimazole: veterinary dose and monograph

At a glance

Drug class

Veterinary drug

Also known as

Vidalta, Neo-Mercazole, Carbimazole

Species with doses

Cats

Routes

PO

Last reviewed

1 October 2026

Indications

• Feline hyperthyroidism — long-term medical management
• Pre-treatment stabilisation before radioiodine therapy or thyroidectomy
• Trial of reversible thyroid control to assess renal function before definitive treatment

Carbimazole 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.

Cats

• Hyperthyroidism (controlled-release): Papich 5e and Plumb’s p.194 (Vidalta label): 15 mg once daily at the same time each day, with food; maintenance 10–25 mg once daily by clinical signs and total T4. Do not split or crush. If less than 10 mg/day is needed, use another treatment.
• Hyperthyroidism (plain carbimazole): Papich 5e and Plumb’s p.194 (Peterson 2000): 5 mg q8h induction, then most cats need 5 mg q12h; 2.5–5 mg q12h (Trepanier). Carbimazole doses are about twice methimazole doses. Titrate to total T4.

Contraindications

Pre-existing significant hepatic disease, blood dyscrasias, or autoimmune disease. Known thioamide (methimazole/carbimazole) hypersensitivity. Pregnancy (crosses placenta — fetal goitre/hypothyroidism). Monitor for the methimazole adverse-effect profile. Handle with care (gloves); pregnant women should avoid handling.

Species-specific warnings

Cats: Monitor CBC and liver values for idiosyncratic toxicity, and renal values as euthyroidism is restored (azotaemia may emerge). Avoid over-treatment into hypothyroidism.

Adverse effects

• GI upset (vomiting, anorexia)
• Facial/neck excoriations (self-trauma — immune-mediated)
• Hepatotoxicity
• Haematologic: thrombocytopenia, neutropenia/agranulocytosis (uncommon but serious)
• Unmasking of underlying CKD as thyroid levels normalise

Drug interactions

• Beta-blockers (atenolol/propranolol): often co-used initially to control tachycardia
• Drugs affected by altered metabolic rate (e.g. those cleared faster when hyperthyroid) — re-evaluate as euthyroidism is restored
• Other bone-marrow-suppressing/hepatotoxic drugs: additive risk

Pregnancy and lactation

Crosses the placenta and can cause fetal goitre/hypothyroidism — avoid in pregnancy. Pregnant human handlers should not handle the tablets.

Monitoring

• Total T4 (efficacy/titration) — recheck ~2–3 weeks after starting/dose changes, then periodically
• CBC (thrombocytopenia/neutropenia) and liver enzymes, especially in the first months
• Renal values (azotaemia may be unmasked as hyperthyroidism is controlled)
• Facial pruritus/excoriation and GI tolerance

Toxicity and overdose

Shares methimazole's adverse profile: GI upset, hepatotoxicity, facial excoriations, and uncommon but serious haematologic toxicity (thrombocytopenia, agranulocytosis). Over-treatment causes iatrogenic HYPOTHYROIDISM (which can worsen renal function). These are idiosyncratic/dose-related effects rather than acute overdose toxicity.

Management: No specific antidote. Discontinue/reduce dose; supportive care; monitor CBC (for cytopenias) and liver values, and check thyroid levels (manage iatrogenic hypothyroidism by dose reduction). Treat hepatotoxicity supportively. Decontaminate large recent ingestions.

Clinical pearls

• Effectively "methimazole, once daily" — carbimazole converts to methimazole, and the controlled-release Vidalta improves compliance over twice-daily methimazole
• It controls but does not cure — radioiodine or thyroidectomy are the definitive options; a medical trial first checks how the kidneys cope with euthyroidism
• Watch for the serious-but-uncommon toxicities (agranulocytosis, hepatotoxicity, facial excoriations) — monitor CBC/liver early on
• Restoring euthyroidism can unmask CKD (GFR falls) — recheck renal values and avoid over-treatment (iatrogenic hypothyroidism worsens kidneys)
• Swallow controlled-release tablets whole; handle with gloves

Mechanism of action

Carbimazole is metabolised to METHIMAZOLE, a thioamide that inhibits thyroid peroxidase — the enzyme that iodinates tyrosine residues and couples them to form thyroid hormones (T4/T3). Blocking this halts new thyroid hormone synthesis (it does not destroy the gland or block release of pre-formed hormone, so the effect takes days to manifest and is fully reversible on discontinuation).

Formulations and products

• Controlled-release 10 / 15 mg tablet (Vidalta)
• 5 mg tablet (Neo-Mercazole, plain)
• Vidalta (MSD Animal Health) — 10 / 15 mg controlled-release tablet, Controlled-release tablet
• Neo-Mercazole — 5 mg tablet, Tablet (human-label)

Storage

Tablets: 15–30 °C, dry, protect from light; handle with gloves (pregnant women should not handle). Swallow controlled-release tablets whole.

Before you use this dose

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.

Other hormones, emergency drugs, supplements and other drugs

• Cabergoline
• Calcitriol
• Calcium Borogluconate (IV Calcium)
• Calcium EDTA (Calcium Disodium Edetate)
• Carboplatin
• Cetirizine
• Chlorambucil
• Chlorpheniramine Maleate

Frequently asked questions