The most common thyroid function tests are:
- Total, or free T4 (the main thyroid hormone in your blood)
- TSH (the hormone from the pituitary gland that stimulates the thyroid to produce T4)
- T3 (the active form of the hormone — T4 is converted to T3)
Other thyroid tests include:
- T3 resin uptake (an older test that is rarely used now)
- Thyroid uptake and scan
- Thyroid binding globulin
- Thyroglobulin
What blood tests do doctors use to check thyroid function?
Doctors may order one or more blood tests to check your thyroid function. Tests may include thyroid-stimulating hormone (TSH), T4, T3, and thyroid antibody tests.
For these tests, a health care professional will draw blood from your arm and send it to a lab for testing. Your doctor will talk to you about your test results.
TSH test
Health care professionals usually check the amount of TSH in your blood first. TSH is a hormone made in the pituitary gland that tells the thyroid how much T4 and T3 to make.
A high TSH level most often means you have hypothyroidism or an underactive thyroid. This means that your thyroid isn’t making enough hormones. As a result, the pituitary keeps making and releasing TSH into your blood.
A low TSH level usually means you have hyperthyroidism or an overactive thyroid. This means that your thyroid is making too much hormone, so the pituitary stops making and releasing TSH into your blood.
If the TSH test results are not normal, you will need at least one other test to help find the cause of the problem.
T4 tests
A high blood level of T4 may mean you have hyperthyroidism. A low level of T4 may mean you have hypothyroidism.
In some cases, high or low T4 levels may not mean you have thyroid problems. If you are pregnant or are taking oral contraceptives NIH external link, your thyroid hormone levels will be higher. Severe illness or using corticosteroids—medicines to treat asthma, arthritis, skin conditions, and other health problems—can lower T4 levels. These conditions and medicines change the number of proteins in your blood that “bind,” or attach, to T4. Bound T4 is kept in reserve in the blood until it’s needed. “Free” T4 is not bound to these proteins and is available to enter body tissues. Because changes in binding protein levels don’t affect free T4 levels, many healthcare professionals prefer to measure free T4.
T3 test
If your health care professional thinks you may have hyperthyroidism even though your T4 level is normal, you may have a T3 test to confirm the diagnosis. Sometimes T4 is normal yet T3 is high, so measuring both T4 and T3 levels can be useful in diagnosing hyperthyroidism.
Thyroid antibody tests
Measuring levels of thyroid antibodies may help diagnose an autoimmune thyroid disorder such as Graves’ disease—the most common cause of hyperthyroidism—and Hashimoto’s disease—the most common cause of hypothyroidism. Thyroid antibodies are made when your immune system attacks the thyroid gland by mistake. Your health care professional may order thyroid antibody tests if the results of other blood tests suggest thyroid disease.
