An adult female just diagnosed with hypothyroidism is started on levothyroxine (Synthroid). What is the most appropriate laboratory test to order for evaluation?

The most appropriate laboratory test to order for evaluation after initiating levothyroxine therapy in a patient with hypothyroidism is thyroid-stimulating hormone (TSH). TSH is a crucial marker in assessing thyroid function and evaluating the efficacy of thyroid hormone replacement therapy.

When a patient is diagnosed with hypothyroidism and begins treatment with levothyroxine, the goal is to normalize thyroid hormone levels in the body. TSH is produced by the pituitary gland in response to the levels of circulating thyroid hormones (T3 and T4). When thyroid hormone levels are low, TSH levels will be elevated, signaling hypothyroidism. As levothyroxine effectively replaces the thyroid hormones, TSH levels should decrease, indicating that the treatment is working appropriately.

Monitoring TSH levels is essential for determining the correct dosage of levothyroxine and ensuring the patient remains within the therapeutic range. Regular assessment of TSH is a critical component of managing hypothyroidism and ensuring optimal care.

Other tests such as follicle-stimulating hormone (FSH) are not relevant to the management of hypothyroidism, as they pertain to reproductive hormones and not thyroid function. Similarly, measuring T3 and T4 alone is not sufficient because TSH provides