Thyroid Hormone Tests

Thyroid hormone tests are blood tests that check how well the thyroid gland is working. The thyroid gland makes hormones that regulate the way the body uses energy.

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. Mental retardation 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 attached to a protein called the thyroxine-binding globulin. Less than 1% of the T4 is unattached. A total T4 blood test measures both bound and free thyroxine. Unattached thyroxine affects tissue function in the body, but attached 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 T4 is present compared to the thyroxine-binding globulin. The FTI can help tell if abnormal amounts of T4 are present because of abnormal amounts of thyroxin-binding globulin.
• Triiodothyronine (T3). Most of the T3 in the blood is attached to the 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.

Thyroid hormone tests are done to:

• Find out what is causing an abnormal thyroid-stimulating hormone (TSH) test. For more information, see the medical test Thyroid-Stimulating Hormone (TSH). This is the most common reason for thyroid hormone tests.
• Check how well treatment for 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 mental retardation, if it is not treated soon after birth.

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 indicate. To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?).

Blood test

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.

Heel stick

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.

Blood test

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.

Heel stick

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.

Blood test

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 (Coumadin), 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.

Heel stick

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.

Normal

Normal values may vary from lab to 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.

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 medical history.

High values

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.

Low values

Low thyroid hormone levels (hypothyroidism) may be caused by:

• Thyroid disease such as thyroiditis.
• Pituitary gland disease.
• Destruction of the thyroid gland by surgery or radiation.

Reasons you may not be able to have the test or why the results may not be helpful include:

• Taking certain medicines, such as:
  • Corticosteroids, estrogen, progesterone, or birth control pills.
  • Blood-thinning medicines such as aspirin, heparin, or warfarin (Coumadin).
  • Antiseizure medicines such as Dilantin or Tegretol.
  • Heart medicines such as amiodarone or propranolol.
  • Lithium.
• Having recently had an X-ray test that uses contrast material.
• Being pregnant.

• 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 medical test Thyroid-Stimulating Hormone.
  • Thyroid antibodies test. This 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 thyroid scan, ultrasound, or biopsy. For more information, see the medical tests Thyroid Scan and Radioactive Iodine Uptake Test, Thyroid Ultrasound and Parathyroid Ultrasound, and Thyroid Biopsy.
• 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.

Citations

1. U.S. Preventive Services Task Force (2008). Screening for congenital hypothyroidism. Available online: http://www.ahrq.gov/clinic/uspstf/uspscghy.htm.

Other Works Consulted

• Chernecky CC, Berger BJ, eds. (2004). Laboratory Tests and Diagnostic Procedures, 4th ed. Philadelphia: Saunders.

• Fischbach FT, Dunning MB III, eds. (2004). Manual of Laboratory and Diagnostic Tests, 7th ed. Philadelphia: Lippincott Williams and Wilkins.

• Handbook of Diagnostic Tests (2003). 3rd ed. Philadelphia: Lippincott Williams and Wilkins.

• Pagana KD, Pagana TJ (2002). Mosby’s Manual of Diagnostic and Laboratory Tests, 2nd ed. St. Louis: Mosby.