Optimal testing for thyroid hormone concentration after treatment with methimazole in healthy and hyperthyroid cats.

BACKGROUND: Methimazole suppresses thyroid hormone synthesis and is commonly used to treat feline hyperthyroidism. The degree of variation in thyroid hormone concentrations 24 hours after administration of methimazole and optimal time for blood sampling to monitor therapeutic efficacy have not been determined. OBJECTIVE: To assess thyroid hormone concentration variation in serum of normal and hyperthyroid cats after administration of methimazole. ANIMALS: Four healthy cats and 889 retrospectively acquired feline thyroid hormone profiles. METHODS: Crossover and retrospective studies. In the crossover study, healthy cats were treated with increasing doses of oral methimazole until steady state of thyroid suppression was achieved. Thyroid hormones and thyroid stimulating hormone (TSH) were serially and randomly monitored after methimazole. Paired t-tests and a 3-factor analysis of variance were used to determine differences between thyroid hormone concentrations in treated and untreated cats in the crossover study. Thyroid profiles from methimazole-treated hyperthyroid cats were retrieved from the Diagnostic Center for Population and Animal Health database and reviewed. Linear regression analysis evaluated relationships of dosage (mg/kg), dosing interval (q24h versus q12h), and time after methimazole to all thyroid hormone concentrations. RESULTS: All serum concentrations of thyroid hormones were significantly suppressed and TSH was significantly increased for 24 hours after administration of oral methimazole in healthy cats (P < .005). In hyperthyroid cats, there were no significant relationships between thyroid hormone concentrations and time postpill or dosing interval. CONCLUSIONS: Timing of blood sampling after oral methimazole administration does not appear to be a significant factor when assessing response to methimazole treatment.