The thyroid gland is a butterfly-shaped gland that lies in front of your windpipe (trachea), just below your voice box (larynx). 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 hormones are needed for normal development of the brain, especially during the first 3 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 (metabolism). The United States Preventive Services Task Force recommends that all newborns be tested for congenital hypothyroidism.1
Thyroid hormone blood tests include:
- 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). 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). 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.
Why It Is Done
Thyroid hormone tests are done to:
- Find out what is causing an abnormal thyroid-stimulating hormone (TSH) test. For more information, see the topic Thyroid-Stimulating Hormone (TSH). This is the most common reason for thyroid hormone tests.
- Check how well 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. A condition called congenital hypothyroidism can prevent normal growth and development and cause other severe problems, such as intellectual disability, if it is not treated soon after birth.
How To Prepare
Many medicines may change the results of this test. Be sure to tell your doctor about all the nonprescription and prescription medicines you take. If you are taking thyroid medicines, tell your doctor when you took your last dose. Your doctor may instruct you to stop taking thyroid medicines temporarily before having this test.
Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?).
How It Is Done
The health professional taking a sample of your blood will:
- Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
- Clean the needle site with alcohol.
- Put the needle into the vein. More than one needle stick may be needed.
- Attach a tube to the needle to fill it with blood.
- Remove the band from your arm when enough blood is collected.
- Put a gauze pad or cotton ball over the needle site as the needle is removed.
- Put pressure on the site and then put on a bandage.
A heel stick is used to obtain a blood sample from a newborn. The baby's heel is pricked with a sharp instrument (lancet) and several drops of blood are collected.
How It Feels
The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.
A brief pain, like a sting or a pinch, is usually felt when the lancet punctures the skin. Your baby may feel a little discomfort with the skin puncture.
There is very little chance of a problem from having blood sample taken from a vein.
- You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
- In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.
- Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin, and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.
There is very little chance of a problem from a heel stick. A small bruise may develop at the site.
Thyroid hormone tests are blood tests that check how well the thyroid gland is working.
The normal values listed here-called a reference range-are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what's normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.
Results are usually available within a few days.
Labs generally measure free T4 (FT4) levels, but also may measure total thyroxine (T4) and T3 uptake (T3U). Results of these thyroid hormone tests may be compared to your thyroid-stimulating hormone (TSH) results.
|Total thyroxine (T4):||
6.4-13.3 mcg/dL (83-172 nmol/L) in babies and older children
5.4-11.5 mcg/dL (57-148 nmol/L) in adults
|Free thyroxine (FT4):|
|Total triiodothyronine (T3):||
105-245 ng/dL (1.6-3.8 nmol/L) in children ages 1-14
82-213 ng/dL (1.3-3.28 nmol/L) in adolescents ages 12-23
80-200 ng/dL (1.2-3.1 nmol/L) in adults
|Free triiodothyronine (FT3):||
260-480 picograms per deciliter (pg/dL) or 4.0-7.4 pmol/L in adults
|Free thyroxine index (FTI):||
1.5-4.5 (index) in adults
Many conditions can change thyroid hormone levels. Your doctor will talk with you about any abnormal results that may be related to your symptoms and past health.
High thyroid hormone levels (hyperthyroidism) may be caused by:
- Diseases of the thyroid gland, such as Graves' disease, thyroiditis, or a goiter that contains one or more abnormal growths (nodules).
- Taking too much thyroid medicine.
What Affects the Test
Reasons you may not be able to have the test or why the results may not be helpful include:
- Taking certain medicines, such as:
- Having recently had an X-ray test that uses contrast material.
- Being pregnant.
What To Think About
- Because false-positive results can occur when testing a newborn for congenital hypothyroidism, the thyroid hormone tests may be repeated a few days after initial testing. If the results are still abnormal and congenital hypothyroidism is suspected, additional testing is done.
- Other blood tests are often used to check how well the thyroid gland is working.
- Thyroid-stimulating hormone (TSH) test measures the amount of TSH in the blood and is considered the most reliable way to find a thyroid problem. If the TSH test is abnormal, other thyroid hormone tests such as a T3 or T4 may be done. For more information, see the topic Thyroid-Stimulating Hormone.
- Thyroid antibodies test measures the presence of antibodies against thyroid tissue. Antibodies may mean that you have an autoimmune disease such as Hashimoto's thyroiditis or Graves' disease.
- Thyroxine-binding globulin (TBG) test. TBG is an important protein in the blood that carries the thyroid hormones T3 and T4. TBG testing is not done very often.
- Other tests used to investigate problems with the thyroid gland include:
U.S. Preventive Services Task Force (2008). Screening for congenital hypothyroidism: Reaffirmation recommendation statement. Available online: http://www.uspreventiveservicestaskforce.org/uspstf08/conhypo/conhyprs.htm.
Other Works Consulted
Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.
Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
Primary Medical ReviewerKathleen Romito, MD - Family Medicine
Specialist Medical ReviewerMatthew I. Kim, MD - Endocrinology
Current as ofNovember 20, 2015