The antithyroid drug with the most rapid onset of antithyroid action is:

(Ref: KDT 6/e p251) Iodides inhibit the secretion of thyroid hormones in the circulation and therefore are the fastest acting antithyroid drugs. Thyroid peroxidase inhibitors are delayed acting because their action manifests only when already stored pool of thyroid hormones is utilized. I131 takes more than 3 weeks to manifest their action.