A 34-year-old woman presents with goiter, tachycardia, and weight loss. TSH is undetectable and free T4 is high. All of the following tests are useful in diagnosing the cause of the hyperthyroidism EXCEPT

• A. FNA biopsy of the thyroid gland
• B. TSH receptor antibodies
• C. RAIU
• D. TSH

Answer: (D)

In hyperthyroidism, the goal is to identify the underlying cause—autoimmune Graves’ disease, a toxic thyroid nodule or multinodular goiter, or thyroiditis—so tests that reveal autoimmune activity, iodine uptake patterns, or nodular pathology are most informative. Here, the patient has biochemical thyrotoxicosis with a suppressed TSH and elevated free T4, so you’d use tests that separate the possible etiologies.

TSH receptor antibodies are helpful because their presence points to Graves’ disease, the common autoimmune cause of diffuse thyroid stimulation. A radioactive iodine uptake (RAIU) scan shows how the thyroid concentrates iodine: diffuse uptake suggests Graves’, while low uptake points to thyroiditis or a non-functioning process, and patchy or focal uptake can indicate toxic nodular disease. FNA biopsy comes into play when there is a suspicious thyroid nodule or a dominant nodule causing thyrotoxicosis or when cancer needs to be ruled out; it provides cytology to guide management.

TSH itself, even though it is the initial screen for thyroid function, does not differentiate the cause of hyperthyroidism. Since TSH is suppressed in virtually all forms of thyrotoxicosis, measuring it again doesn’t illuminate the underlying etiology.

So the test that does not contribute to identifying the cause is the measurement of TSH.