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Abstract:  The efficacy of transdermal methimazole in 16 cats with hyperthyroidism was evaluated. During the 2-year study period, 15 of the 16 treated cats exhibited a significant decrease in serum T4 value, and in 1 cat the serum T4 level increased. The mean total dose, which was administered once or twice daily, was 6.53 mg/day (range, 5 to 12.5 mg). An initial recheck of the T4 level was conducted after an average of 7.4 weeks (range, 4 to 38 wk) of treatment with transdermal methimazole. The cats that responded to therapy showed a mean reduction in the T4 level from 10.82 mg/dL (range, 4.2 to 24 mg/dL) to 2.76 mg/dL (range, 0.3 to 5.2 mg/dL). The only adverse effect of treatment (an increase in the blood urea nitrogen [BUN] level) developed in only one cat (6%) within 30 days of the initiation of therapy; that patient was scheduled to undergo reevaluation of the BUN level after 2 months. The increase in the BUN level in that cat, which was 16 years of age, was probably due to an unmasking of preexisting senile renal dysfunction rather than treatment-induced toxicity. The most common symptoms of the 16 cats studied before they underwent treatment with transdermal methimazole were weight loss, which was associated with increased appetite in some cats, and heart murmur. Other symptoms included vomiting, vocalization, and lethargy. The results of the study indicate that transdermal methimazole is effective in blocking excess thyroid hormone secretion in most cats and that it is a relatively safe antithyroid medication therapy for use in cats.

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