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Understanding thyroid blood tests

Last Modified: December 07, 2023

Family Medicine, Diseases & Disorders


The thyroid is a butterfly-shaped gland that lies in front of your windpipe, just below your voice box. The thyroid makes hormones that control the way your body uses energy and can affect the function of systems throughout the body. If your provider suspects your thyroid might not be making hormones in the proper amounts, they might order a thyroid hormone blood test to check how well your thyroid gland is working. Let’s learn more about this important gland and the different types of thyroid blood tests.

What does the thyroid gland do?

The thyroid gland uses iodine from food to make two thyroid hormones: thyroxine (T4) and triiodothyronine (T3). The thyroid gland stores these thyroid hormones and releases them as they are needed.  Thyroid blood tests are done if your doctor suspects that your thyroid is making too much thyroid hormone (hyperthyroidism) or not enough hormone (hypothyroidism).

Thyroxine (T4) is the most common form of thyroid hormone that is released by the thyroid gland. Once it reaches the tissues, it is converted to another form of thyroid hormone called triiodothyronine (T3), which is more active in regulating the body's chemical reactions.

In tests of thyroid function, a low T4 level may indicate the presence of low thyroid levels (hypothyroidism). Certain forms of liver or kidney disease may also cause a low T4 level, even though a thyroid problem may not be present.

Thyroid hormones are also needed for normal development of the brain, especially during the first three years of life. Intellectual disability may occur if a baby's thyroid gland does not produce enough thyroid hormone (congenital hypothyroidism). Older children also need thyroid hormones to grow and develop normally, and adults need the hormones to regulate the way the body uses energy. The United States Preventive Services Task Force recommends that all newborns be tested for congenital hypothyroidism.

Why are thyroid hormone tests done?

Thyroid hormone tests are done to:

  • Find out what is causing an abnormal thyroid-stimulating hormone (TSH) test. This is the most common reason for thyroid hormone tests.
  • Check how well the treatment of thyroid disease is working. The total thyroxine (T4), free thyroxine (FT4), and free thyroxine index (FTI) values are often used to keep track of treatment for hyperthyroidism.
  • Screen newborns to find out if the thyroid gland function is normal.
  • Check a child’s thyroid gland function. Thyroid hormone tests are blood tests that check to see how well your child's thyroid gland is working.

Types of thyroid blood tests

Four main types of tests can be done on your blood to check various levels of hormones associated with the thyroid. These include:

Thyroid-stimulating hormone (TSH) test

A thyroid-stimulating hormone (TSH) blood test is used to check for thyroid gland problems. TSH is produced when the hypothalamus releases a substance called thyrotropin-releasing hormone (TRH). TRH then triggers the pituitary gland to release TSH. TSH causes the thyroid gland to make two hormones: triiodothyronine (T3) and thyroxine (T4). T3 and T4 help control your body's metabolism. This test may be done at the same time as tests to measure T3 and T4.

Total thyroxine (T4)

Most of the thyroxine (T4) in the blood is bound to a protein called thyroxine-binding globulin. Less than 1% of the T4 is free. A total T4 blood test measures both free and bound thyroxine. Free thyroxine affects tissue function in the body, but bound thyroxine does not.

Free thyroxine (FTI or FT4) test

Free thyroxine (T4) can be measured directly (FT4) or calculated as the free thyroxine index (FTI). The FTI tells how much free T4 is present compared to bound T4. The FTI can help tell if abnormal amounts of T4 are present because of abnormal amounts of thyroxine-binding globulin.

Triiodothyronine (T3) test

Most of the T3 in the blood is attached to thyroxine-binding globulin. Less than 1% of the T3 is unattached. A T3 blood test measures both bound and free triiodothyronine. T3 has a greater effect on the way the body uses energy than T4, even though T3 is normally present in smaller amounts than T4.

Making sense of your lab test involves more than just knowing the numbers. When receiving lab test results always consult with your provider. They will look for any patterns of abnormal lab results and can tell you what your test results mean for you and your health.

A thyroid profile is available through Parkview’s direct access testing (DAT). DAT allows you to self-order your laboratory tests and get your results. This convenient service does not require a physician visit or order and provides an out-of-pocket option for those without health insurance or who prefer not to file a claim through their insurance. To learn more, visit

To learn more about Parkview’s laboratory services, visit us here.














Copyrighted material adapted with permission from Healthwise, Incorporated. This information does not replace the advice of a doctor.


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