Thyroid Function Tests

What are thyroid function tests?

Thyroid function tests are common tests done to see how well the thyroid is working. Some of the most common thyroid function tests include:

• Blood tests. This test is done to measure the level of thyroid-stimulating hormone (TSH) in the blood, which helps determine thyroid function. High levels of the hormone indicate an underactive thyroid. Very low levels indicate an overactive thyroid. A blood test can also measure levels of hormones thyroxine (T4) and triiodothyronine (T3), and less often, a protein called thyroglobulin.

• Ultrasound exam of the thyroid gland. An ultrasound scan uses high-frequency sound waves to create an image of the internal organs. It's done to detect signs of abnormal enlargement, or nodules, on the thyroid, and to guide biopsies of concerning nodules. 

• Thyroid scans using radioactive iodine or technetium (another radioactive element). This test helps determine the cause of abnormal thyroid function. This may be important in deciding whether a person will need his or her thyroid removed (thyroidectomy).

• Radioactive iodine uptake test. This test involves giving a small amount of radioactive iodine to measure how much is taken up by the thyroid. This test can distinguish between several causes of elevated thyroid hormone blood levels.