Transdermal
methimazole is suggested as an alternative to oral
therapy for
hyperthyroid cats that are difficult to pill. However, no information on long-term management with this treatment is available. Our objective was therefore to retrospectively evaluate the efficacy and safety of long-term transdermal
methimazole treatment in
hyperthyroid cats. Sixty cats with newly diagnosed
hyperthyroidism and available long-term follow-up information were included.
Methimazole was formulated in a
pluronic lecithin organogel-based vehicle and was applied to the pinna of the inner ear. Cats were re-evaluated at regular intervals. Median (range) follow-up was 22.6 months (3.6-88.4 months). Clinical improvement was observed in all cats and side effects were rare (mild transient gastrointestinal signs: n = 3;
erythema of the pinna: n = 2, necessitating a switch to oral medication). Despite a significant decrease, with median T4 concentrations within the reference interval during the follow-up period, several cats repeatedly had T4 concentrations in the thyrotoxic and hypothyroid range. Maximal and minimal daily doses during the follow-up period were 15.0 and 1.0 mg, respectively; they were significantly higher than the starting dose after 24-36 months of
therapy. Although the majority of owners were highly satisfied with the treatment, several admitted not treating their cat regularly. Transdermal
methimazole is a safe option for the long-term management of feline
hyperthyroidism. However, it seems difficult to keep the T4 concentrations constantly within the reference interval. Higher doses can be expected after prolonged treatment and, despite the convenience of transdermal application, owner compliance should be assessed regularly.