Which assessment should be made after initiating methimazole therapy in a patient with Graves' disease?

After initiating methimazole therapy in a patient with Graves' disease, monitoring thyroid-stimulating hormone (TSH) levels is essential. Methimazole functions by inhibiting the synthesis of thyroid hormones, which can lead to a decrease in circulating thyroid hormones over time. As treatment progresses, a subsequent reduction in thyroid hormone levels will trigger a compensatory increase in TSH production from the pituitary gland. Monitoring TSH levels helps determine the effectiveness of the therapy and assesses whether the patient's thyroid function is moving toward a euthyroid state. Elevated TSH levels indicate that the therapy may need adjustments, while low TSH levels may suggest that the dose is adequate or that the patient is becoming hyperthyroid again.

Thyroid antibodies levels and free T4 levels can provide additional information regarding the patient's condition, but TSH levels are a more direct measure of the feedback loop between the thyroid and the pituitary gland, making it the most critical assessment immediately following the initiation of therapy. Thyroid ultrasounds are typically used to evaluate structural changes rather than functional status and are not routinely monitored after starting medication.