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Transdermal methimazole in hyperthyroid cats

Sep 26, 2014

Boretti FS, Sieber-Ruckstuhl NS, Schafer S, et al.  Transdermal application of methimazole in hyperthyroid cats:  a long-term follow-up study.  Journal of Feline Medicine and Surgery 2014; 16:453-9.

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Cats can be difficult to medicate orally, so other routes of administration of commonly used essential medications that are easier for caregivers to administer are very desirable.   This retrospective study of 60 client-owned cats with newly diagnosed, naturally occurring hyperthyroidism presented to a university small animal clinic in Europe assessed the effectiveness, adverse side effects, and owner compliance during long-term management of hyperthyroidism in these patients.  Criteria for inclusion in the study included clinical signs consistent with hyperthyroidism and a T4 concentration > 3.5 µg/dL. The study population included 26 male castrated and 34 female spayed cats, including 49 European shorthairs and 11 pedigreed cats of various breeds; the median age was 14 years.

All patients included in the study were treated with methimazole formulated as 5 mg/0.1 mL in a pluronic  lecithin organogel (PLO) vehicle by the same compounding pharmacy;  PLO is a polymeric surfactant which enhances the formation of drug micelles in a gel matrix.  Starting dose of the methimazole was 2.5-5.0 mg/cat administered in one dose (q 24 hours) or two divided doses (q 12 hours).  The cats were re-evaluated regularly with history, physical examination, serum blood urea nitrogen (BUN), creatinine, and T4 concentrations.  Other testing such as complete biochemical profile, CBC, and urinanalysis were performed if the cat was azotemic or the owners reported polyuria and polydipsia.  Follow-up continued for up to 84 months, until the patient died, was lost to follow-up, or to the end of the study in 2012.  By the end of the study, 19 cats were still alive, 2 were lost to follow-up, and 39 had been euthanized for known causes including renal failure, heart failure, neoplasia, as well as unknown causes (n = 10).  Median survival time of the euthanized cats was 21.9 months (range 3.6-76.8 months).  Most owners (n=58) were satisfied with the ease of application of the medication, but 10 owners admitted not using the gel regularly, and 4 stopped the treatment for varying periods of time.

The transdermal methimazole therapy was well tolerated and led to a significant decrease in serum T4 concentration in all 60 cats shortly after initiating therapy.  Two of the cats developed inflammation and erythema of the pinna, but after prolonged use of the gel (over one year).  Median serum T4 concentrations were within the reference range at all rechecks, but  a number of cats repeatedly had T4 concentrations that would “ping-pong” between the thyrotoxic and hypothyroid ranges, and some of this was probably attributable to owner compliance issues.  These variations were, however, also found in cats whose owners claimed to be treating them consistently; after 24-36 months  the cats required daily doses of methimazole higher than their original starting doses to maintain T4 within the reference range.  This may be due to the possibility that thyroid adenomas can enlarge despite medical treatment and eventually transform into thyroid carcinomas, or inconsistencies in the formulation of the PLO-based methimazole gel.  None of the cats developed overt acute renal decompensation during the study, although 28% of the patients demonstrated increases in serum BUN and creatinine above the reference range during the study, which is comparable with that described in the literature during anti-thyroidal medical therapy.  No mention was made of whether patients treated with methimazole twice daily were better and more consistently regulated, or had greater or fewer side effects, than those receiving methimazole every 24 hours, but an earlier publication (see below) by the same authors may elaborate on this. [PJS]

See also:
Boretti FS, Sieber-Ruckstuhl NS, Schafer S., et al.  Duration of T4 suppression in hyperthyroid cats treated once and twice daily with transdermal methimazole.  J Vet Intern Med 2013;27:377-81.


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