What are thyroid function tests?

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

• Blood tests. One blood test is done to measure the level of thyroid-stimulating hormone (TSH) in the blood. This helps see how well the thyroid is working. High levels of the TSH indicate an underactive thyroid. Very low levels indicate an overactive thyroid. Other blood tests may be done as well. These include measuring the levels of the hormones made by the thyroid itself. These hormones are called thyroxine (T4) and triiodothyronine (T3). A blood test to measure the level of thyroid antibodies may also be done. These are used to determine the cause and help predict the future course of thyroid problems.

• Ultrasound. This test uses high-frequency sound waves to create an image of the internal organs. It's done to find signs of abnormal enlargement, or nodules, on the thyroid. It's also used to guide biopsies of nodules. 

• Thyroid scan. This test helps find the cause of abnormal thyroid function. This may be important in deciding whether you will need your 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 tell the difference between several causes of increased thyroid hormone blood levels.