A 30-year-old woman is found to have a low serum thyroxine level after being evaluated for fatigue. Five years ago she was treated for Graves' disease with radioactive iodine. The diagnostic test of choice is

TSH levels are always increased in patients with untreated hypothyroidism (from primary thyroid disease) and would be the test of choice in this patient. Serum T3 is not sensitive for hypothyroidism. The TRH stimulation test is used to assess pituitary reserve of thyroid-stimulating hormone. A decreased RAIU is of limited value because of the low value for the lower limit of normal. In goitrous hypothyroidism, the RAIU may even be increased.