Transdermal application of methimazole in hyperthyroid cats: a long-term follow-up study

Abstract

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.

Effect of Feeding an Iodine‐Restricted Diet in Cats with Spontaneous Hyperthyroidism

“…Lack of decrease in heart rate in our study could simply be a product of variability in environment and each individual cat's demeanor. Treatment with both I 131 and thyroid peroxidase inhibitors (methimazole, carbimazole) increased body weight in treated cats . As weight loss is thought to occur in hyperthyroidism because of increased basal metabolic rate from excess thyroid hormone circulation, normalization of TT4 concentration in the cats of this study should have led to an increase in body weight.…”

Effects of 6 Weeks of Parenteral Cobalamin Supplementation on Clinical and Biochemical Variables in Cats with Gastrointestinal Disease

“…Treatment differed among cats and included dietary changes (7/20), probiotics (7/20), prednisolone (7/20), transdermal methimazole (4), chlorambucil (3/20), and antibiotics (3/20).…”

Feline Hyperthyroidism

“…Methimazole for transdermal application is usually dispensed in tuberculin syringes and can be formulated in a variety of concentrations (typically 2.5 or 5 mg/0.1 mL). In a long-term study of 60 cats treated with transdermal methimazole at doses ranging from 1 to 15 mg per day, clinical improvement was seen in all cats although higher doses were required after prolonged treatment, and several cats repeatedly had T 4 concentrations above or below the reference range during the study (Boretti et al, 2013b). Dosing is alternated between ears and, if necessary, residual gel is removed from the ear prior to the next application using a cotton ball.…”