Clinical Efficacy of Transdermal Methimazole in Cats with Hyperthyroidism.

Sartor L.L., Trepanier L.A., Kroll M., et al.

Conference Proceedings, (2003). American College of Veterinary Internal Medicine: p.393


Methimazole (Tapazole) is the drug of choice for the medical management of feline hyperthyroidism in the United States. Recently, custom veterinary pharmacies have offered methimazole in a PLO gel formulation, containing lecithin, isopropyl palmitate, and pluronic acid, for transdermal administration in cats. The purpose of this study was to determine whether transdermal methimazole was as safe and effective as oral methimazole for the control of naturally occurring hyperthyroidism in cats. Forty-seven cats with newly diagnosed hyperthyroidism were enrolled in the study. Cats were randomized to receive either transdermal methimazole in PLO (applied to the inner pinna), or oral methimazole (2.5 mg BID for either route). Cats were evaluated at weeks 0, 2, and 4, with a physical exam, body weight, CBC, biochemical panel, UA, total T4, indirect Doppler blood pressure, and owner questionnaire. Data at each evaluation was compared between groups using Chi Square or unpaired t-test, as appropriate; data over time was evaluated by repeated measures ANOVA followed by FLSD test. 44 cats completed the protocol (17 oral and 27 transdermal). There were no significant differences between groups at enrollment regarding T4, methimazole dosage (mg/kg) or other clinical parameters. Significantly more cats treated with oral methimazole were euthyroid after 2 weeks (87.5%) compared to those treated by the transdermal route (56.0%; P 5 .027); however, this difference was no longer significant by 4 weeks of treatment (81.8% euthyroid for oral versus 66.7% euthyroid for transdermal). Cats treated with oral methimazole had a higher incidence of GI side effects (23.5%) compared to the cats treated with topical methimazole (3.7%; P 5 .04), but there were no differences between groups in the incidence of leukopenia, hepatotoxicity, or facial excoriations. The results of this study indicate that transdermal methimazole in PLO gel results in comparable efficacy by 4 weeks of treatment, with fewer GI side effects, compared to the same dose administered orally, and is therefore is a viable alternative in the treatment of feline hyperthyroidism.