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 micro +/- g/dL) and 4 weeks (3.2 vs 1.7 microg/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.