Hyperthyroidism is the most common endocrine disorder in middle-aged and older cats. Medical management with the antithyroid drug methimazole has become a popular option, either before surgery (thyroidectomy) or radioiodine treatment, or for long-term medical management. Carbimazole is rapidly metabolized to methimazole in vivo. This cross-over study of 6 normal cats compared a single oral dose of either a controlled-release carbimazole tablet or a sugar-coated methimazole tablet. Both formulations are approved for use in cats and were found to be well tolerated by the cats in the course of the study. In general, the pharmacokinetic values for the two treatments were similar when differences in effective methimazole dose were taken into consideration. Both drugs were reported to have relatively short plasma half-lives in the current study that would seem to support either once or twice daily dosing for either. Methimazole accumulates in the thyroid gland so that antithyroid effects extend beyond (possibly up to 24 hours) the duration of elevated plasma levels. Therefore, the short half-life of 3 hours for both controlled-release carbimazole and sugar-coated methimazole tablets is consistent with current practice recommendations for divided daily dosing, although once-daily dosing may be appropriate for maintenance therapy in some cats. [VT]
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