Thyroid Hormone Synthesis Inhibitors

Thyroid peroxidase carries out the iodination and coupling of the hormonogenic tyrosine residues of thyroglobulin, the reaction that actually builds T3 and T4. Drugs in this class block that enzyme: the FDA-approved veterinary label for methimazole states that its primary action is to inhibit the binding of iodide to thyroid peroxidase, preventing the catalysed iodination of thyroglobulin and the synthesis of T3 and T4. Because it stops production rather than removing stored hormone, the clinical effect appears only after the existing thyroid store is used up - the reason feline hyperthyroidism takes weeks to control.