Patients who are treated with antithyroid drugs are usually monitored with serial measurement of thyroid function tests. When treatment is first started, thyroid function tests are usually checked to determine whether the dose of methimazole or propylthiouracil that has been prescribed is high enough to inhibit the production and secretion of excess amounts of thyroid hormone. If adjustment of a dose is necessary, thyroid function tests are usually checked at an appropriate interval after each increase or decrease. Once an effective dose of methimazole or propylthiouracil has been confirmed, thyroid function tests may be checked at regular intervals to monitor a patient's response to treatment.
Thyroid function tests that are checked to monitor responses to treatment may vary over time, depending on the stage of treatment a patient taking antithyroid drugs has reached. While TSH levels usually provide the most sensitive indication of whether there is an adequate amount of thyroid hormone present in the body, they may not provide accurate information during the first few months of treatment with antithyroid drugs. Long-term exposure to excess amounts of thyroid hormone may suppress the function of the cells that produce TSH in the pituitary gland to a point where it may take a period of several weeks for them to recover. During this period, TSH levels may remain suppressed, even after the amount of thyroid hormone in the body has returned to a normal level. For this reason, the amount of thyroid hormone in the body is usually measured directly during the first few months of treatment with antithyroid drugs. This usually involves checking thyroid function tests that measure T4 levels, free T4 levels, or T3 levels. Over time, as the cells that produce TSH in the pituitary gland gradually recover, TSH levels may be checked to monitor a patient's response to treatment.