Efficacy and safety of once versus twice daily administration of methimazole in cats with hyperthyroidism

Lauren A. Trepanier Department of Medical Sciences, School of Veterinary Medicine, University of Wisconsin, Madison, WI 53706.

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 DVM, PhD, DACVIM, DACVCP
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Stacey B. Hoffman Department of Medical Sciences, School of Veterinary Medicine, University of Wisconsin, Madison, WI 53706.
Present address is North Florida Veterinary Specialists, 3444 Southside Blvd, Ste 102, Jacksonville, FL 32216.

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 MS, DVM
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Mandy Kroll CatCare Clinic, 601 N Whitney Way, Madison, WI 53705.

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Ilona Rodan CatCare Clinic, 601 N Whitney Way, Madison, WI 53705.

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 DVM, DABVP
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Laura Challoner CatCare Clinic, 601 N Whitney Way, Madison, WI 53705.

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Abstract

Objective—To determine whether once daily administration of methimazole was as effective and safe as twice daily administration in cats with hyperthyroidism.

Design—Randomized, nonblinded, clinical trial.

Animals—40 cats with newly diagnosed hyperthyroidism.

Procedure—Cats were randomly assigned to receive 5 mg of methimazole, PO, once daily (n = 25) or 2.5 mg of methimazole, PO, twice daily (15). A complete physical examination, including measurement of body weight; CBC; serum biochemical analyses, including measurement of serum thyroxine concentration; and urinalysis were performed, and blood pressure was measured before and 2 and 4 weeks after initiation of treatment.

Results—Serum thyroxine concentration was significantly higher in cats given methimazole once daily, compared with cats given methimazole twice daily, 2 weeks (3.7 vs 2.0 μg/dL) and 4 weeks (3.2 vs 1.7 μg/dL) after initiation of treatment. In addition, the proportion of cats that were euthyroid after 2 weeks of treatment was lower for cats receiving methimazole once daily (54%) than for cats receiving methimazole twice daily (87%). Percentages of cats with adverse effects (primarily gastrointestinal tract upset and facial pruritus) were not significantly different between groups.

Conclusions and Clinical Relevance—Results suggest that once daily administration of methimazole was not as effective as twice daily administration in cats with hyperthyroidism and cannot be recommended for routine use. (J Am Vet Med Assoc 2003; 222:954–958)

Abstract

Objective—To determine whether once daily administration of methimazole was as effective and safe as twice daily administration in cats with hyperthyroidism.

Design—Randomized, nonblinded, clinical trial.

Animals—40 cats with newly diagnosed hyperthyroidism.

Procedure—Cats were randomly assigned to receive 5 mg of methimazole, PO, once daily (n = 25) or 2.5 mg of methimazole, PO, twice daily (15). A complete physical examination, including measurement of body weight; CBC; serum biochemical analyses, including measurement of serum thyroxine concentration; and urinalysis were performed, and blood pressure was measured before and 2 and 4 weeks after initiation of treatment.

Results—Serum thyroxine concentration was significantly higher in cats given methimazole once daily, compared with cats given methimazole twice daily, 2 weeks (3.7 vs 2.0 μg/dL) and 4 weeks (3.2 vs 1.7 μg/dL) after initiation of treatment. In addition, the proportion of cats that were euthyroid after 2 weeks of treatment was lower for cats receiving methimazole once daily (54%) than for cats receiving methimazole twice daily (87%). Percentages of cats with adverse effects (primarily gastrointestinal tract upset and facial pruritus) were not significantly different between groups.

Conclusions and Clinical Relevance—Results suggest that once daily administration of methimazole was not as effective as twice daily administration in cats with hyperthyroidism and cannot be recommended for routine use. (J Am Vet Med Assoc 2003; 222:954–958)

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