Development of the newer, sensitive assay for thyrotropin (thyroid-stimulating hormone [TSH]) has simplified the diagnosis and management of hypothyroidism. Serum free thyroxine (T4) levels measured by analogue techniques or free T4 index may be used in conjunction with TSH test results to confirm the diagnosis. Treatment consists of daily doses of levothyroxine sodium (Levothroid, Levoxine, Synthroid), which are increased slowly to achieve an optimal dose. Close monitoring with TSH assay is important to ensure adequate treatment. In euthyroid sick syndrome, results of thyroid function tests are abnormal in patients with nonthyroidal illness; treatment is indicated if symptoms of hypothyroidism are present. Patients with subclinical hypothyroidism also may benefit from treatment.