Serum Concentrations of Methimazole in Cats after a Single Oral Dose of Controlled-Release Carbimazole or Sugar-Coated Methimazole (Thiamazole)

Longhofer S., Martin-Jimenez T. and Soni-Gupta J.

Vet Ther, 2010. 11(3): p.E1-7.


Methimazole (thiamazole) is an antithyroid drug commonly used to treat feline hyperthyroidism. It is routinely given twice daily. Carbimazole is a methimazole derivative that is rapidly metabolized to methimazole in vivo. A controlled-release tablet for once-daily carbimazole therapy has recently been developed in an attempt to improve compliance during medical management of feline hyperthyroidism. The results of a crossover study in six cats suggest that the pharmacokinetics of methimazole with a single dose of this controlled-release tablet may be similar to those with a single dose of a sugar-coated methimazole tablet when the two drugs are given at an equimolar dose. The mean half-lives were nearly identical (3.12 hours, sugar-coated methimazole tablets; 3.28 hours, controlled-release carbimazole tablets). The serum concentrations of methimazole at 24 hours were 21.7 +/- 28.9 ng/mL in the cats treated with 5-mg sugar-coated methimazole tablets and 28.7 +/- 37 ng/mL in the cats treated with 10-mg carbimazole tablets (which provide approximately 25% more methimazole after conversion to the active metabolite