Oral as well as topical administration has been shown to be effective in normalizing serum thyroid hormone levels in hyperthyroid cats. After normalization of thyroid hormone concentrations, glomerular filtration rate may decrease, potentially exacerbating or 'unmasking' underlying renal disease. If renal parameters improve or remain static after a methimazole treatment trial, a more permanent treatment option may be pursued safely. Additionally, even if renal parameters worsen slightly but the clinical status of the cat improves because of remission of the hyperthyroidism and no clinical signs of renal disease/failure develop, definitive therapy can be sought.
In cats, the plasma half-life of methimazole ranges from less than 3 hours to approximately 6 hours. Methimazole's activity correlates with intrathyroidal drug concentrations, not the plasma half-life. Because methimazole, like other thioureylenes, is concentrated in thyroidal tissue, the biological effect of the drug exceeds its plasma half-life.
Various recommendations have been made regarding initial dosing of methimazole, ranging from 2.5 mg to 15 mg/day in divided doses of every 8-12 hours. Most cats require between 5 and 10 mg/day of methimazole to control their disease; twice or three times daily administration is the most effective. Once an effective dose has been reached, T4 levels usually fall within the low normal reference range.
While effective in most cases, methimazole administration is associated with side effects in up to 18% of treated cats (Peterson et al, 1988). The most common side effects are vomiting and anorexia, but more serious side effects necessitating discontinuation of the drug occur in up to 10% of cats. These include self-induced excoriation, hepatotoxicity, thrombocytopenia and agranulocytosis. In cats that cannot undergo surgical thyroidectomy or radioiodine therapy, there are limited options.
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