Comparison of the disposition of carbimazole and methimazole in clinically normal cats

The oral disposition of the antithyroid drugs methimazole and carbimazole were compared in nine clinically normal cats. After the administration of 5 mg of methimazole, serum concentrations of methimazole increased in all the cats, with mean drug concentrations reaching peak values (1.37 micrograms ml-1) at 30 minutes. After administration of 5 mg carbimazole, serum concentrations of carbimazole remained low, but serum methimazole became readily measurable, with mean drug concentrations reaching peak values (0.79 microgram ml-1) at 120 minutes. When serum concentrations of methimazole attained after administration of the two antithyroid drugs were compared, the mean maximum serum methimazole concentration achieved after administration of methimazole was approximately twofold higher than peak concentrations measured after administration of carbimazole. In addition, the mean area under the serum concentration curve (AUC) after administration of methimazole was approximately twofold higher than the mean AUC determined after administration of carbimazole. When the differences in molecular weight between the two drugs was taken into consideration, however, these methimazole:carbimazole ratios of 2:1 were nearly equivalent to the molar ratio of the 5 mg doses of the drugs given (1.63). Results of this study indicate that carbimazole is nearly totally converted to methimazole after oral administration to cats, similarly to the findings in man. The finding of less available serum methimazole after administration of a 5 mg tablet of carbimazole than after methimazole is also consistent with published antithyroid drug dosages needed to control hyperthyroidism in cats.