Carbimazole
**Carbimazole** is a thioimidazole-derivative **anti-thyroid** medication primarily used in veterinary medicine to manage **feline hyperthyroidism**. * It acts as a **prodrug** and is rapidly converted in vivo to **methimazole**. * Because it lacks the bitter taste of methimazole, it is often better tolerated by cats, reducing the incidence of gastrointestinal side effects. * While not available in the USA or Canada, it is widely used in Europe and other regions. * **Clinical Pearl**: Carbimazole requires approximately twice the milligram dose of methimazole to achieve equivalent serum levels due to differences in molecular weight.
Mechanism: Carbimazole is a prodrug that is almost entirely converted to **methimazole** in the body. * Methimazole actively inhibits the enzyme **thyroid peroxidase (TPO)**. * Inhibition of **TPO** → blocks the oxidation of iodide to iodine → prevents iodine incorporation into tyrosyl residues of **thyroglobulin**. * It also inhibits the coupling of iodinated tyrosyl residues (monoiodotyrosine and diiodotyrosine) → prevents formation of **thyroxine (T4)** and **triiodothyronine (T3)**. * *Note*: It does not affect the release or activity of pre-formed thyroid hormones already in circulation, which accounts for the 1-3 week lag time before clinical improvement is seen.
Dosing by species
- Hyperthyroidism · 15 mg/animal (starting dose) or 10 mg/animal if total thyroxine <100 nmol/l. Adjust dose in 5 mg increments. · PO · q24h · Long-term · Do not break tablets. Note: There are no data on the use of the sustained-release formulation in dogs.
- Hyperthyroidism · 2.5-5 mg (total dose) per cat · PO · twice daily
- Hyperthyroidism · 10-15 mg total dose daily per cat in divided doses · PO · divided doses · for 1-3 weeks · Will produce a euthyroid state for most patients. Then adjust dosage for the patient to the lowest effective dose. Most cats will need dosing at least once daily.
- Hyperthyroidism · Initially, give 5 mg (total dose) · PO · q8h · for 2-3 weeks · Then adjust. May need to increase dose in approximately 10% of cats. Most cats require 5 mg PO q12h to maintain euthyroidism.
- Hyperthyroidism (Sustained-release tablets) · 15 mg · PO · once daily · Give at the same time each day. Do not break or crush tabs. Adjust dose upwards or downwards within a range of 10 mg-25 mg per day in 5 mg increments depending on clinical signs and TT4. If cat requires doses less than 10 mg per day, use alternative treatment.
- Hyperthyroidism · 15 mg/animal (starting dose) or 10 mg/animal if total thyroxine <100 nmol/l. Adjust dose in 5 mg increments. · PO · q24h · Long-term · Do not break tablets.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Routes of administration
Contraindications
- Hypersensitivity to carbimazole or methimazole
- Not recommended in cats intolerant to methimazole
- History of or concurrent hematologic abnormalities
- Liver disease
- Autoimmune disease
Adverse effects
- Vomiting
- Anorexia
- Depression
- Eosinophilia (usually transient)
- Leukopenia (usually transient)
- Lymphocytosis (usually transient)
- Self-induced excoriations (rare but serious)
- Bleeding (rare but serious)
- Hepatopathy (rare but serious)
- Thrombocytopenia (rare but serious)
- Agranulocytosis (rare but serious)
- Positive direct antiglobulin test (rare but serious)
- Acquired myasthenia gravis (rare but serious)
- Positive ANA (with chronic use)
Drug interactions
- Bupropion · Potential for increased risk for hepatotoxicity; increased monitoring (LFT's) necessary
- Digoxin · Carbimazole may decrease digoxin efficacy
- Warfarin · Potential for decreased anticoagulant efficacy if carbimazole added
- Iodine-131 (Radioiodine) · Carbimazole interferes with the uptake and efficacy of radioiodine therapy. It must be discontinued prior to treatment. · major
- Low iodine prescription diets · Concurrent use is not recommended as it may interfere with the dietary management of hyperthyroidism. · major
Monitoring
- CBC, platelet counts (baseline and every 2-3 weeks during first 3 months)
- Serum T4 (baseline and every 2-3 weeks during first 3 months)
- Liver function tests (if indicated by clinical signs)
- ANA (if indicated by clinical signs)
- T4 at 3-6 month intervals (after stabilized)
- Other diagnostic tests as dictated by adverse effects
Overdose
Acute toxicity that may be seen with overdosage includes vomiting, anorexia, depression, and more serious effects such as **agranulocytosis**, **hepatopathy**, and **thrombocytopenias**. > **Treatment**: Consists of following standard protocols in handling an oral ingestion (empty stomach if not contraindicated, administer charcoal, etc.) and treating symptomatically and supportively.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.