Duration of T4 Suppression in Hyperthyroid Cats Treated Once and Twice Daily with Transdermal Methimazole

Abstract

Background: Transdermal methimazole is an acceptable alternative to oral treatment for hyperthyroid cats. There are, however, no studies evaluating the duration of T4 suppression after transdermal methimazole application. Such information would be valuable for therapeutic monitoring.Objective: To assess variation in serum T4 concentration in hyperthyroid cats after once-and twice-daily transdermal methimazole administration.Animals: Twenty client-owned cats with newly diagnosed hyperthyroidism. Methods: Methimazole was formulated in a pluronic lecithin organogel-based vehicle and applied to the pinna of the inner ear at a starting dose of 2.5 mg/cat q12h (BID group, 10 cats) and 5 mg/cat q24h (SID group, 10 cats). One and 3 weeks after starting treatment, T4 concentrations were measured immediately before and every 2 hours after gel application over a period of up to 10 hours.Results: Significantly decreased T4 concentrations were observed in week 1 and 3 compared with pretreatment concentrations in both groups. All cats showed sustained suppression of T4 concentration during the 10-hour period, and T4 concentrations immediately before the next methimazole treatment were not significantly different compared with any time point after application, either in the BID or SID groups.Conclusions: Because transdermal methimazole application led to prolonged T4 suppression in both the BID and SID groups, timing of blood sampling does not seem to be critical when assessing treatment response.

Feline Hyperthyroidism

“…Another novel lipophilic formulation has also been reported to result in effective absorption of methimazole (Hill et al, 2011). Other studies have confirmed that transdermal methimazole therapy can be very effective for long-term treatment of feline hyperthyroidism at doses ranging from 2.5 mg every 24 hours to 5 mg every 12 hours (Hill et al, 2011;Boretti et al, 2013a). The gel is applied to the non-haired pinna of the ear by the owners using a finger-cot.…”

“…The gel is applied to the non-haired pinna of the ear by the owners using a finger-cot. 4-35) (Boretti et al, 2013a). In a study of 47 cats with newly diagnosed hyperthyroidism, randomized to receive either oral or transdermal methimazole, significantly more cats treated with oral methimazole were euthyroid than those treated with transdermal methimazole after 2 weeks of treatment, but by 4 weeks the difference was no longer significant.…”

Feline hyperthyroidism: diagnosis and treatment

“…It begins after two to three weeks and includes measurement of a complete blood count, alkaline phosphatase, alanino-aminotransferase, blood urea nitrogen concentration and creatinine concentration, along with total thyroxine blood concentrations (Trepanier, 2007). Timing of blood sampling is not a significant factor for total thyroxine blood concentration measurement (Rutland et al, 2009;Borreti et al, 2013). If side effects are noted, the drug should be discontinued and another therapeutic option should be pursued (Luyre, 2006).…”

Diagnosis and management of feline hyperthyroidism: current perspectives

“…73 The recommended initial dose of the transdermal formulation is 5-10 mg per cat, and once-daily application appears to be as effective in controlling total T 4 as twice-daily application. 74,75 Gastrointestinal side effects were found to occur less frequently with the transdermal formulation than with the oral formulation. 73 Additional adverse effects associated with the transdermal formulation include mild crusting and erythema at the site of application.…”