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July/August 2014 Today's Veterinary Practice 39 FOCUS ON PHARMACOLOGY | MeTHiMAzOLe: MANAGeMeNT OF FeLiNe HYPeRTHYROidiSM tvpjournal.com nant or lactating queens; laborator y studies in rats and mice have shown evidence of teratogenic and embr yotoxic effects of this medication. IN THE LITERATURE A number of recent studies have evaluated the use of methimazole in cats with hyperthyroidism, evalu- ating its effects on renal and thyroid function, route of administration, and quality of life. Best Use of Antithyroid Drugs Hypertension, progression of chronic kidney dis- ease, iatrogenic hypothyroidism, and persistence of hyperthyroidism are all concerns when manag- ing patients with hyperthyroidism. A recent paper (2014) described the "best practice" of using antithy- roid drugs for pharmacologic management of hyper- thyroid cats. 4 Treatment. Two drugs have been li- censed for cats in the last decade: me- thimazole and its prodrug carbimazole. Based on current evidence and avail- able tablet sizes, recommended starting doses include: • Methimazole: 2.5 mg PO Q 12 H • Carbimazole (sustained release formulation): 10 to 15 mg PO Q 24 H . These doses should then be titrated to effect in order to obtain circulating total thyroxine (TT4) concentra- tions in the lower half of the reference interval. Monitoring. Patients should be monitored for side effects, especially during the first months of treat- ment. Some side effects may require discontinuation of treatment. At each monitoring visit, clinical condition and quality of life should also be evaluated, with special attention to possible development of azotemia, hypertension, and iatrogenic hypothyroidism. When euthyroidism has been achieved, monitoring visits are recommended after 1 month, 3 months, and twice yearly thereafter. Survival Time. Cats with pre-existing azotemia have shorter survival times. However, development of mild azotemia during the initial course of treatment, unless associated with hypothyroidism, does not appear to de- crease survival time. Long-Term Effects. The long-term ef- fects of chronic medical management re- quire further study, including the value of monitoring free T4 (f T4) and thyroid stimulating hormone (TSH) concentra- tion to detect subclinical hyper- and hy- pothyroidism, respectively. 4 Owner Experiences with Management A recent study (2013) surveyed 111 owners of hyperthy- roid cats about their experiences and views on the man- agement of hyperthyroidism. 5 Transdermal Methimazole: How Does It Measure Up? A recent pharmacokinetic study looked at a novel lipophilic formulation of methimazole—pluronic lecithin organogel (PLO)—for transdermal use and compared it with oral carbimazole. 3 in the first 24 hours: • Cats treated with 5 mg methimazole transdermally did not have reliably detectable serum concentra- tions of methimazole • Cats treated with 5 mg carbimazole orally or 10 mg methimazole transdermally had detectable serum concentrations of methimazole. Compared with cats receiving 5 mg oral carbimazole, those receiving 10 mg methimazole transdermally had a: • Lower maximum concentration and area under the curve • Longer maximal concentration and elimination half-life • Higher mean concentration in serum at 148 hours. The mean relative bioavailability of 10 mg transdermal methimazole compared to oral carbimazole was 48% (min, 43%; max, 55%). table. CoMParison oF transderMal MethiMazole & oral CarbiMazole MEDICATION TIME AT MAX CONCENTRATION ELIMINATION HALF-LIFE MEAN CONCENTRATION (148 HOURs) Carbimazole: 5 mg oral 2.1 (± 1.6) hours 5.1 (± 1.2) hours 255 (± 28) ng/mL Methimazole: 5 mg transdermal n/a n/a 204 (± 76) ng/mL Methimazole: 10 mg transdermal 5.2 (± 1.1) hours 13 (± 3) hours 506 (± 165) ng/mL