Thyroid Function Tests

What are thyroid function tests?

Thyroid function tests are common procedures performed to determine how well the thyroid is functioning. Some of the most common thyroid function tests include the following:

• Blood tests. 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. In addition, a blood test can measure levels of hormones thyroxine (T4) and triiodothyronine (T3), and to measure a protein called thyroglobulin.

• Ultrasound exam of the thyroid gland. To detect signs of abnormal enlargement, or nodules, on the thyroid, and to guide biopsies of concerning nodules. (An ultrasound scan is a diagnostic technique that uses high-frequency sound waves to create an image of the internal organs.)

• Thyroid scans using radioactive iodine or technetium (a radioactive metallic element). To help determine the cause of abnormal thyroid function. This is important in deciding whether a person will need a total thyroidectomy or partial thyroidectomy.

• Functional stimulation tests. Tests that help differentiate whether the problem is due to the pituitary gland, the hypothalamus, or the thyroid gland. One such test includes injecting thyrotropin-releasing hormone (produced by the hypothalamus) and measuring the pituitary's response.