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Thyroid Hormone Testing

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;23:1025-1030.

Hyperthyroidism is one of the most common endocrinopathies of cats and its prevalence is increasing in recent years. Methimazole is commonly used to treat hyperthyroidism and its action is to suppress thyroid hormone synthesis. The optimal time for blood sampling to monitor therapeutic efficacy, and the amount of variation in thyroid hormone concentrations 24 hours post administration of methimazole have not been determined. In this study, healthy cats were given steadily increasing doses of oral methimazole until a steady state of thyroid suppression was achieved. It was found that there is a significant and sustained suppression of total thyroxine (TT4), total tri-iodothyronine (TT3), free T3, and free T4 concentrations for 24 hours after a once daily methimazole administration in healthy cats. The time to thyroid gland suppression in healthy cats was 2-5 weeks. In addition, a retrospective study of 889 thyroid profiles from methimazole-treated hyperthyroid cats was reviewed. The review noted that in hyperthyroid cats, there was no significant relationship between thyroid hormone concentrations and time post-pill or dosing interval. The conclusion was timing of blood sampling after oral methimazole administration does not appear to be a significant factor when assessing response to treatment. [VT]
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Related articles:
Trepanier L, Hoffman S, Kroll M, et al. Efficacy and safety of once versus twice daily administration of methimazole in cats with hyperthyroidism. J Am Vet Med Assoc 2003;222:954-958.
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Trepanier LA. Pharmacologic management of feline hyperthyroidism. Vet Clin North Am Small Anim Pract 2007;37:775-788, vii.
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