Hyperthyroidism is the most common endocrine disorder of older cats. If left untreated, excessive production of thyroxine (T4) and triiodothyronine (T3) can lead to detrimental systemic effects and eventual death. The 3 treatment options for feline hyperthyroidism are pharmacologic therapy (methimazole), surgical ablation of the thyroid, and radioactive iodine therapy. Bioavailability of orally administered methimazole varies from 27% to 100%, with an average of 80%. Side effects, which usually occur within the first 3 months of therapy, are common. A reported 15% of patients treated with oral methimazole have vomiting, anorexia, lethargy, and hematologic changes. Facial excoriations and hepatopathy have been observed in 2% of cases. A transdermal preparation of methimazole, using a pleurolecithin organogel, has recently become available. This study was performed to investigate the pharmacodynamic effects and clinical safety of transdermal methimazole in hyperthyroid cats.

Thirteen newly diagnosed hyperthyroid cats were treated with 5 mg (0.1 ml of a 50-mg/ml preparation) methimazole applied to alternating internal ear pinnae Q 12 H for 28 days. Baseline hematologic and biochemical values as well as serum T4 levels were measured at day 0 and again during rechecks on days 14 and 28. Ten cats completed the study. Two cats were euthanized for unrelated reasons, and 1 cat was withdrawn from the study after developing thrombocytopenia and severe facial erythema on the internal pinnae of both ears. Clinical improvement and a significant decrease in T4 were noted in all cats. None of the cats showed the gastrointestinal side effects seen with oral methimazole. One third of the owners noted a nonhomogenous texture to the gel, caused by drug precipitation, which probably caused a variation in drug concentration from 1 dose to the next. Most owners were happy with the ease of gel administration, but only 2 chose to continue with topical therapy on a long-term basis because of the cost and lack of stability of the product. The authors concluded that transdermal methimazole is a safe and effective option for treating hyperthyroid cats, but because of cost, lack of stability, and a dearth of pharmacokinetic studies, its use should be reserved for short-term treatment or for treatment of cats that cannot or will not tolerate oral administration. The authors recommend pharmacokinetic studies be performed on hyperthyroid cats receiving transdermal methimazole to determine serum level variability and bioavailability as well as long-term stability studies to ensure that constant concentrations of methimazole are maintained.

COMMENTARY: Despite the ability to permanently resolve hyperthyroidism via surgery or radioiodine therapy, there are times when these options are not feasible due to anesthetic risk, inaccessibility to radioiodine treatment, or concurrent renal failure. Cats requiring treatment with methimazole, however, are not always cooperative when it comes to administering oral medication. The increasing availability of transdermal methimazole for these patients has been beneficial, but as this study shows, some problems still need to be worked out. A recent pharmacokinetic study of transdermal methimazole in normal cats showed only 2 of 6 cats with detectable serum levels of methimazole. While hyperthyroid cats (as in this study) appear to respond positively to transdermal methimazole, pharmacokinetic studies have yet to be done. Until these studies are done and the other issues that are raised here are addressed, clinical use of this preparation may, unfortunately, be limited.

Clinical efficacy and safety of transdermal methimazole in the treatment of feline hyperthyroidism. Lecuyer M, Prini S, Dunn ME, Doucet MY. CAN VET J 47:131-135, 2006.