Today's Veterinary Practice

JUL-AUG 2014

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| FOCUS ON PHARMACOLOGY Today's Veterinary Practice July/August 2014 40 Treatment. The final treatment decision was usually based on the veterinarian's recommendation or joint deci- sion-making between the owner and veterinarian. • Oral antithyroid medication was offered to 92% of owners. • Almost all cats (103/111; 93%) had received oral antithy- roid medication at some point during the course of their disease. • At survey completion, 69 cats (62%) were receiving oral antithyroid medication. Results. Management of hyperthyroidism using United Kingdom veterinary-licensed oral antithyroid medication (me- thimazole or carbimazole) was associated with 72% to 75% success rates in terms of owner-assessed clinical outcome. • The most important treatment priorities for owners were: » Prescription of the most accurate dose of medication » Use of lowest possible dose. • No owners ranked once daily treatment as most impor- tant. • Over three quarters (79%) of owners said that they were, or would be, happy to dose their cats twice daily to control hyperthyroidism. • For 62% of owners, pilling their cats twice daily was not a problem. Conclusion. These results suggest that, for most cat owners, there is no barrier to prescribing twice-daily anti- thyroid medication, if required. 5 Transdermal Methimazole Treatment A retrospective study (2013) was conducted to evaluate the efficacy and safety of long-term transdermal methimazole treatment in hyperthyroid cats. Sixty cats with newly diag- nosed hyperthyroidism and available long-term follow-up information were included. 6 Treatment & Monitoring. Methimazole was formulat- ed in a PLO-based vehicle and applied to the pinna of the inner ear. Depending on clinician preference, the starting doses were: • 2.5 to 5 mg/cat administered in 1 dose (Q 24 H) or • 2.5 to 5 mg/cat administered in 2 divided doses (Q 12 H) . Cats were re-evaluated at regular intervals, and median follow-up was 22.6 months. Results. Clinical improvement was observed in all cats and side effects were rare (mild transient gastrointestinal signs, n = 3; erythema of the pinna, n = 2), but necessi- tated a switch to oral medication. • Several cats repeatedly had T4 concentrations in the thyrotoxic and hypothyroid range, despite a significant decrease in overall median T4 concentrations into the reference interval during the follow-up period. • After 24 to 36 months of therapy, maximal and minimal daily doses during the follow-up period were 15 and 1 mg, respectively, of which the former is significantly higher than the starting dose. • Although the majority of owners were highly satisfied with treatment, several admitted not treating their cats regularly. Conclusion. The authors concluded that transdermal methimazole is a safe option for the long-term manage- ment of feline hyperthyroidism. However, it seems diffi- cult to maintain T4 concentrations consistently within the reference interval. A requirement for higher doses can be expected after prolonged treatment and, despite the con- venience of transdermal application, owner compliance should be assessed regularly. 6 ADMINIsTRATION IN FELINE PATIENTs Dosage Starting doses for methimazole therapy have decreased since the disease was first discovered, mainly due to the lower concentrations of TT4 seen in the majority of hyper- thyroid patients diagnosed today. The suggested initial starting doses are: • Methimazole: 2.5 mg PO Q 12 H or 2.5 mg transder- mal Q 12 H • Carbimazole (sustained release): 10 to 15 mg PO Q 24 H . Route Transdermal methimazole is suggested as an alternative to oral therapy for hyperthyroid cats that are difficult to pill. See Transdermal Methimazole Treatment, for further information on transdermal administration. Duration Methimazole can be used: • Short term to control TT4 concentrations prior to more definitive therapy (radioactive iodine or surgery) • Long term for medical management, with appropriate monitoring (see MONITORING, page 41). Adverse Effects Common. In cats, the most common side effects of methimazole/carbimazole administration are gastroin- testinal (hyporexia to anorexia, vomiting, and diarrhea). Most of the gastrointestinal side effects can be controlled by discontinuation of the medication and supportive care. Once the signs have resolved, the medication can be restarted at a lower dose and titrated upward to achieve the desired clinical and biochemical endpoints. Severe. More severe side effects include lymphadenop- athy, 7 hepatopathies, aplastic anemia, thrombocytopenia, and agranulocytosis, which are generally manifested within the first few months of treatment. Uncommon. Less frequent events include facial pruritus, exfoliative dermatitis, myasthenia gravis, 8 and a bleeding disorder secondary to vitamin K antagonism. Drug Interactions • Anticoagulants may be potentiated by the antivitamin K activity of methimazole. • Decreased clinical efficacy of phenobarbital if the drugs are used concurrently. • A reduction in dose of certain drugs (alpha adrenergic blocking agents, digitalis glycosides, and theophyl- line) may be needed when the patient becomes euthy- roid. • Methimazole is known to reduce the hepatic oxidation of benzimidazole anthelmintics (eg, fenbendazole), leading to increased plasma concentration of these anthelmintics when administered concurrently.

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