Drug Monograph
Full clinical overview, indications, dosage references & safety notes.
Overview
Carbimazole (Vidalta®) is an oral antithyroid medication used in cats for the management of hyperthyroidism. It is a prodrug that is rapidly and almost completely converted in vivo to methimazole, the active metabolite responsible for its therapeutic effects.
Methimazole inhibits thyroid peroxidase and interferes with iodine incorporation into tyrosyl residues of thyroglobulin, thereby reducing thyroid hormone synthesis. It also prevents coupling of iodinated tyrosyl residues to form iodothyronines. Carbimazole does not affect the release or activity of thyroid hormones already formed or circulating in the bloodstream.
Carbimazole is rapidly absorbed from the gastrointestinal tract. Administration with food increases bioavailability. Because of molecular weight differences, oral carbimazole doses must be approximately twice those of methimazole to achieve equivalent serum concentrations. After oral dosing, plasma elimination half-life of methimazole in cats ranges from approximately 2.3 to 10.2 hours, although biologic effects may persist longer due to thyroid tissue accumulation.
A lag time of approximately 1–3 weeks is typically observed before significant reductions in serum total T4 concentrations occur. Prolonged-release formulations may allow once-daily dosing. Treatment controls excessive thyroid hormone production but does not cure the underlying disease.
Indications
Carbimazole is used in cats for the medical management of hyperthyroidism and its associated clinical signs. It is considered an agent of choice for controlling excessive thyroid hormone production.
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Feline hyperthyroidism:
Used to reduce serum thyroid hormone concentrations and control clinical signs such as weight loss, tachycardia, hypertension, and hyperactivity. -
Stabilization prior to definitive therapy:
May be administered before radioiodine treatment or surgery to achieve a euthyroid state. -
Long-term medical management:
Appropriate for cats in which radioiodine therapy or thyroidectomy is not selected or is contraindicated.
Dosage (Reference)
Cat
Carbimazole is administered orally for management of hyperthyroidism. Tablets must be given whole and should not be crushed or split. Administer with food.
| Clinical use | Route | Dose | Frequency | Notes |
|---|---|---|---|---|
| Hyperthyroidism (initial therapy) | PO | 15 mg/animal | Once daily (q24h) | Use 10 mg/animal once daily if total T4 <100 nmol/L at baseline. |
• Adjust dose in 5 mg increments based on clinical response and laboratory values.
• Do NOT break or crush prolonged-release tablets.
• Dosing requirements may change over time.
• There is typically a 1–3 week lag before significant reductions in serum T4 are observed.
Warnings & Precautions
Carbimazole should be used cautiously in cats due to the potential for hematologic, hepatic, renal, and immune-mediated adverse effects. Careful case selection and regular monitoring are essential.
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Hypersensitivity:
Contraindicated in cats hypersensitive to carbimazole or methimazole. Cats with prior severe reactions to methimazole should not receive carbimazole. -
Concurrent systemic disease:
Do not use in cats with severe primary liver disease, diabetes mellitus, or signs of autoimmune disease. -
Hematologic abnormalities:
Contraindicated in cats with abnormal red or white blood cell counts, thrombocytopenia, coagulopathies, anemia, neutropenia, or lymphopenia. -
Renal function:
Treatment of hyperthyroidism may unmask underlying renal disease due to decreased glomerular filtration rate once euthyroidism is restored; monitor renal parameters closely. -
Hepatic disease:
Use with caution in cats with preexisting hepatic disease. -
Progression of thyroid disease:
Hyperthyroidism may be progressive despite medical control; enlargement or multifocal thyroid disease may occur over time. -
Hazardous drug handling:
Methimazole (active metabolite) is classified by NIOSH as a hazardous drug. Wash hands after handling and use appropriate personal protective equipment (PPE). -
Medication errors:
Do not confuse carbiMAZOLe with carbamazEPINE or CARBOplatin. Avoid use of unclear abbreviations in prescriptions.
Drug Interactions
The following interactions have been reported or are theoretical in cats receiving carbimazole (or its active metabolite, methimazole). When used concurrently, assess risk–benefit and monitor closely.
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Anticoagulants (e.g., heparin, rivaroxaban):
Methimazole may potentiate anticoagulant activity; monitor for bleeding abnormalities. -
Warfarin:
Methimazole has antivitamin K activity and may alter clotting factor metabolism; increased monitoring of anticoagulant effect is warranted. -
Benzimidazole antiparasitics (e.g., fenbendazole, albendazole, thiabendazole):
May reduce hepatic oxidation of benzimidazoles and increase blood concentrations of parent drug or metabolites. -
Beta-adrenergic antagonists (e.g., atenolol, propranolol):
Dose adjustments may be required once the patient becomes euthyroid. -
Chloramphenicol:
Concurrent use may increase risk of myelosuppression. -
Dipyrone:
May increase risk of myelosuppression when used together. -
Digoxin:
Restoration of euthyroidism may reduce digoxin clearance, increasing risk of toxicity; dose reduction may be needed. -
Phenobarbital:
May reduce the clinical effectiveness of carbimazole. -
Prednis(ol)one:
Dose reduction may be required once euthyroid state is achieved. -
Theophylline:
Dose reduction may be needed once euthyroid state is achieved. -
Iodine-131:
No difference in response to radioiodine has been noted based on timing of methimazole discontinuation relative to I131 administration.
Side Effects & Overdose
Side Effects
Most adverse effects associated with carbimazole (or methimazole) therapy in cats occur within the first 3 months of treatment. Many are dose-related and may resolve with dose adjustment, but some require drug discontinuation.
- Vomiting, anorexia, lethargy: Most common adverse effects (≈10%); often transient and may improve with dosage reduction.
- Hematologic changes: Eosinophilia, leukopenia, and/or lymphocytosis (≈5%); usually transient but discontinue if severe or persistent. Rarely, agranulocytosis or thrombocytopenia may occur.
- Hepatopathy: May develop and requires discontinuation if significant liver dysfunction occurs.
- Immune-mediated disorders: Rare cases of immune-mediated hemolytic anemia, immune-mediated thrombocytopenia, or acquired myasthenia gravis have been reported.
- Self-induced excoriations: Facial pruritus and scratching may necessitate withdrawal of therapy.
- Bleeding tendencies: May occur secondary to thrombocytopenia or coagulopathies.
- Renal parameter changes: Treatment may unmask underlying renal dysfunction as hyperthyroidism is controlled.
Overdose
Acute toxicity following overdose is expected to reflect exaggerated adverse effects. Severity depends on dose and duration of exposure.
- Severe hematologic effects: Agranulocytosis, thrombocytopenia, or immune-mediated hemolytic anemia.
- Gastrointestinal effects: Vomiting or GI bleeding (including possible hematemesis).
- Hepatitis or nephritis: May occur in severe cases.
- Dermatologic changes: Erythema, alopecia, or coat abnormalities.
- Management: Follow standard protocols for oral overdose (e.g., gastric decontamination if appropriate, activated charcoal) and provide supportive care based on clinical signs.
- Consultation: In suspected overdose cases, consultation with a veterinary poison control center is recommended.
Key Notes
Practical clinical considerations for safe and effective use of carbimazole in cats with hyperthyroidism:
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Prodrug concept:
Carbimazole is converted almost completely to methimazole in vivo; oral carbimazole doses are approximately twice those of methimazole to achieve equivalent serum concentrations. -
Lag to clinical effect:
A reduction in serum total T4 typically requires 1–3 weeks after initiation of therapy. -
Long-term therapy:
Medical treatment controls hyperthyroidism but does not cure it; therapy is often required for the remainder of the cat’s life unless definitive treatment is pursued. -
Structured monitoring schedule:
During the first 3 months, perform physical examination, blood pressure measurement, CBC (including platelets), serum chemistry, and TT4 at baseline and after 3 and 6 weeks or after dosage changes. Once stable (≥3 months), recheck TT4 every 3–6 months. -
TSH and fT4 interpretation:
Serum TSH may assist in identifying iatrogenic hypothyroidism; free T4 should not be used as the sole monitoring parameter. -
Fructosamine considerations:
Hyperthyroidism and its treatment may affect serum fructosamine; defer interpretation for diabetic control assessment until hyperthyroidism has been controlled for at least 6 weeks. -
Administration instructions:
Give with food and administer exactly as prescribed to maintain stable thyroid control.
