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

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 mug/dL) and 4 weeks (3.2 vs 1.7 mug/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.