Dose titration

To avoid overtreatment, consider titrating the dose down after 4 to 6 weeks (or earlier if treatment was started at a particularly high dose) according to clinical and biochemical response. Serum TSH concentration can remain suppressed for several months after correction of hyperthyroidism, so base initial dose adjustment on serum T₃ and T₄ concentrations. If the thyroid hormone concentrations drop by half, the dose can be halved.

Continue to assess clinical and biochemical response every 4 to 6 weeks, adjusting the dose of the antithyroid drug to achieve thyroid hormone concentrations in the normal range. Hyperthyroidism can persist despite normalisation of serum T₄ concentration due to persistent elevation of T₃. Consider increasing the dose of the antithyroid drug in these patients, particularly if they are still symptomatic.

Once the patient is euthyroid, carbimazole can be given once daily as maintenance therapy. Propylthiouracil has a shorter plasma half-life and tissue action, so should continue in divided doses.

Do not stop the antithyroid drug abruptly when thyroid hormone concentrations have normalised; see Subsequent management of primary hyperthyroidism for management after starting antithyroid drug therapy.

Antithyroid drug therapy is usually effective; inadequate response may indicate nonadherence. Patients exposed to marked iodine excess (eg contrast media, topical iodine preparations, amiodarone, complementary medicines [particularly those containing kelp]) may be resistant to antithyroid drugs and often require a higher dose.