When should you test hyperthyroid cats after methimazole administration?

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As small-animal clinicians know, hyperthyroidism is a common disorder in cats. Treatment often includes management with methimazole, a drug blocking thyroid hormone synthesis. Monitoring the response to treatment is important, and veterinarians often have questions about the timing of thyroid hormone testing. In a recent study published in the Journal of Veterinary Internal Medicine, researchers evaluated the optimal timing for blood sampling to monitor the efficacy of methimazole.

This study involved analysis of thyroid hormone concentrations from healthy cats treated with methimazole as well as thyroid hormone profiles from methimazole-treated hyperthyroid cats. The healthy cats were treated with increasing oral doses of methimazole until thyroid suppression was achieved. After reaching sustained thyroid suppression, a blood sample was taken at four-hour increments after treatment on six consecutive days.

The profiles of the hyperthyroid cats were divided based on once-a-day and twice-a-day dosing. Hyperthyroid cats with concurrent illnesses were included in the study. Since other disease processes can suppress thyroid hormone concentrations, some test results may have been decreased by concurrent disease rather than methimazole treatment alone. However, since many hyperthyroid cats have additional medical conditions, they were considered to be a good representation of the hyperthyroid cat population.

Surprisingly, the results of this study showed that in both healthy and hyperthyroid cats, there was no relationship between thyroid hormone concentrations and time after drug administration or dosing interval. Even when the profiles were divided into once- and twice-a-day treatment groups, there was no connection between the time following drug administration and thyroid hormone concentrations. This suggests that the effect of methimazole is sustained beyond its expected half-life (2.3 hours in hyperthyroid cats). More important, the findings show that the timing of blood sampling after oral methimazole administration is not a significant factor when assessing response to treatment.

Link to abstract: http://www3.interscience.wiley.com/journal/122544388/abstract

Rutland BE, Nachreiner RF, Kruger JM. Optimal testing for thyroid hormone concentration after treatment with methimazole in healthy and hyperthyroid cats. J Vet Intern Med 2009;23(5):1025-1030.