Cat Hyperthyroid Medical Management: Methimazole and Carbimazole

Antithyroid drugs provide effective medical management of feline hyperthyroidism. Understanding dosing, monitoring and side effects enables optimal welfare outcomes.

Key Facts

Methimazole and carbimazole are the primary antithyroid medications for feline hyperthyroidism
Carbimazole is a prodrug that converts to methimazole after absorption
Side effects occur in 10-20% of cats and include facial pruritus, vomiting and blood dyscrasias
T4 should be checked 2-4 weeks after starting treatment and every 3-6 months once stable
Renal function must be monitored as hyperthyroidism masks concurrent kidney disease

Welfare Considerations

Medical management of hyperthyroidism is reversible and allows assessment of renal function before committing to curative treatment. Cats with mild side effects may benefit from transdermal methimazole applied to the ear pinna. Severe side effects including thrombocytopenia, agranulocytosis or facial excoriation require drug withdrawal. Monthly blood counts in the first 3 months identify haematological side effects early. Renal disease unmasked after euthyroidism is achieved may require reducing the antithyroid dose to balance thyroid and renal function.

What You Can Do

Give methimazole or carbimazole at the prescribed dose at consistent times each day
Monitor for side effects including facial itching, vomiting and lethargy in the first weeks
Request T4, renal function and blood count testing 4 weeks after starting treatment
Report any facial scratching or vomiting promptly as these may indicate drug reaction
Discuss renal reassessment timing with your vet once euthyroidism is achieved