Vetdrugs 2.0Carbimazole is a thionamide antithyroid drug and a prodrug of methimazole. Its principal veterinary use is medical control of feline hyperthyroidism, either for continued management or to establish a euthyroid state before definitive treatment. It suppresses new hormone production while functioning thyroid tissue remains present, rather than destroying that tissue.
Group
Antithyroid prodrug
Mechanism of Action
Carbimazole is converted to methimazole, which inhibits thyroid-peroxidase-dependent reactions within the thyroid follicle. This reduces oxidation and incorporation of iodide into tyrosyl residues of thyroglobulin and inhibits coupling of iodotyrosines to form triiodothyronine and thyroxine. As stored hormone is depleted, circulating thyroid hormone concentrations decline and the metabolic effects of hyperthyroidism diminish.
Dose Rates
Cats
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Hyperthyroidism — Initial | 5 mg/animal | PO | Every 8 h | Immediate-release carbimazole. |
| Hyperthyroidism — Maintenance | 5 mg/animal | PO | Every 12 h | Immediate-release carbimazole. |
| Hyperthyroidism with sustained-release tablets | Reference only | PO | Vidalta sustained release: 15 mg per cat once daily for induction, then 10–25 mg once daily adjusted to thyroid results. Do not substitute immediate-release tablets in this schedule. | |
| Hyperthyroidism | 2.5–5 mg/animal | PO | Every 12 h | |
| Divided-dose induction | Reference only | PO | 10–15 mg total per cat per day orally in divided doses for 1–3 weeks, followed by the lowest effective maintenance amount; division count is not fixed. |
Indications
- Long-term medical control of hyperthyroidism in cats.
- Preoperative stabilization of hyperthyroid cats before thyroidectomy.
- Temporary control of feline hyperthyroidism while awaiting radioiodine treatment or other definitive management.
Contraindications
- Bone marrow suppression or thrombocytopenia.
- Hypersensitivity to carbimazole or methimazole; prior serious methimazole reaction requires great caution.
- Hepatic disease or autoimmune disease requires additional assessment.
Interactions
- Digoxin or warfarin response may change as thyroid status normalizes; reassess concurrent treatment.
- Bupropion has a theoretical additive hepatic-toxicity interaction.
Adverse effects
- Vasculitis or lupus-like reactions
- Bone marrow suppression, leukopenia or agranulocytosis
Preparations
| Preparation | Form | Strength | Route |
|---|---|---|---|
| Carbimazole 5 mg immediate-release tablet | Tablet | 5 mg / tablet | PO |
Monitoring and Administration
Monitor thyroid hormones and CBC, and titrate to clinical response. Approximately 10 mg carbimazole corresponds to 6 mg methimazole; switching requires dose reassessment.
Immediate-release and sustained-release regimens are distinct. Transdermal compounded carbimazole has poor stability.
During the first three months, assess CBC, platelets and T4 at baseline and every 2–3 weeks. Once stable for at least three months, T4 checks every 3–6 months are described. Add liver tests if signs suggest toxicity.
Sustained-release tablets must be swallowed whole at a consistent time daily. The 10–25 mg range uses combinations of 10 and 15 mg tablets; do not split them. A requirement below 10 mg/day calls for a different treatment formulation.