Thyroid Hormone Tests

Test Overview

Thyroid hormone tests are blood tests that evaluate the function of the thyroid gland. Thyroid hormones play a key role in normal development of the brain, especially during the first 3 years of life. A baby whose thyroid gland does not produce enough thyroid hormone (congenital hypothyroidism) may, in severe cases, become mentally retarded. Older children also need thyroid hormones to grow and develop normally, and adults need the hormones to regulate their metabolism.

The thyroid 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.

- Thyroxine (T4) is produced by your thyroid gland when the pituitary gland releases thyroid-stimulating hormone.

- Only a small amount of triiodothyronine (T3) is produced directly by your thyroid gland. Most T3 is made by other tissues that convert T4 into T3.

Thyroid hormone blood tests include:

- Free thyroxine index (FTI or FT4). Free thyroxine (T4) can be measured directly (FT4) or calculated (FTI). The FTI is a measure of the amount of T4 in relation to the amount of thyroxine-binding globulin present. The FTI is calculated from the T4 and T3 uptake, or T3U, values. The FTI value can indicate when an abnormal level of T4 is caused by an abnormal level of thyroxin-binding globulin in the blood.

- Triiodothyronine (T3). T3 has a greater effect on metabolism than T4, even though T3 is normally present in lower amounts than T4. The total amount of T3 in the blood or the amount of free T3 (FT3) can be measured. Normally, less than 1% of the T3 is free.

- Triiodothyronine uptake (T3U). The T3U test is an indirect measurement of the amount of the protein (thyroxine-binding globulin) that can bind to T3 and T4. The results of this test are useful only when evaluated along with other thyroid function tests.

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

For newborns, the blood sample is obtained by heel stick.

Why It Is Done

Thyroid hormone tests are done to:

a) Evaluate the cause of 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.

b) Monitor the effect of treatment for your thyroid disease, such as Graves' disease. The total thyroxine (T4), free thyroxine (FT4), and free thyroxine index (FTI) values are often used to monitor treatment for hyperthyroidism.

c)Screen newborns for underactive thyroid gland function, a condition called congenital hypothyroidism. This condition can interfere with normal growth and development and can cause other severe problems (such as mental retardation) if it is not discovered soon after birth.

How To Prepare

No special preparation is required before having this test.

Many medications may affect thyroid hormone tests but not necessarily affect your thyroid-stimulating hormone (TSH) level. Inform your health professional about all prescription and nonprescription medications you are taking. If you are taking thyroid medications, tell the health professional when you took your last dose. Your health professional may instruct you to stop taking thyroid medications temporarily before having this test. For more information on TSH, see the What to Think About section of this topic.

Talk to your health professional about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will indicate. Complete the medical test information form to help you understand the importance of the test.

How It Is Done

Blood sample from a vein
The health professional drawing blood will wrap an elastic band around your upper arm to temporarily stop the flow of blood through the veins in your arm. This makes it easier to put a needle into a vein properly because the veins below the band get larger and do not collapse easily.

The needle site is cleaned with alcohol and the needle is inserted. More than one needle stick may be needed if the needle does not get placed correctly or if your vein cannot supply enough blood.

When the needle is properly placed in your vein, a collection tube will be attached to the needle and blood will flow into it. Sometimes more than one tube of blood is collected.

When enough blood has been collected, the band around your arm will be removed. A gauze pad or cotton ball is placed over the puncture site as the needle is withdrawn. Pressure is applied to the puncture site for several minutes and then a small bandage is often placed over it.

Blood sample from a 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.

How It Feels

Blood sample from a vein
You may feel nothing at all from the needle puncture, or you may feel a brief sting or pinch as the needle goes through the skin. Some people feel a stinging pain while the needle is in the vein. However, many people do not feel any pain (or have only minor discomfort) once the needle is positioned in the vein. The amount of pain you feel depends on the skill of the health professional drawing the blood, the condition of your veins, and your sensitivity to pain.

Blood sample from a heel stick
A baby usually feels a brief sting or pinch when the lancet pricks the skin for a heel stick. While the blood is being collected, there is little or no discomfort.


Blood sample from a vein
There is very little risk of complications from having blood drawn from your vein. You may develop a small bruise at the puncture site. You can reduce the risk of bruising by keeping pressure on the site for several minutes after the needle is withdrawn.

Rarely, your vein may become inflamed after the blood sample is taken. This condition is called phlebitis and is usually treated by applying a warm compress several times daily.

Continued bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medications can also make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medication, tell the health professional before your blood is drawn.

Blood sample from a heel stick
There is very little risk of complications from having blood drawn from a heel stick. A small bruise may develop at the puncture site. You can reduce the risk of bruising by keeping pressure on the site for several minutes after the blood is collected.


Normal values may vary from lab to lab. Labs generally measure free T4 (FT4) levels, but may measure total thyroxine (T4) and T3 uptake (T3U) as well. Results of these thyroid hormone tests may be compared to your thyroid-stimulating hormone (TSH) results. For more information on TSH tests, see the What to Think About section of this topic.

Thyroid hormone tests

Total thyroxine (T4):
- 4.9 micrograms per deciliter (µg/dL) in newborns (1 to 5 days of age)
- 5–12 µg/dL in adults

Free thyroxine (FT4): 0.9–2.4 nanograms per deciliter (ng/dL)

Total triiodothyronine (T3): 70–195 ng/dL

Free triiodothyronine (FT3): 0.2–0.6 ng/dL

Free Thyroxine Index (FTI): 4–11

Greater-than-normal values may mean:
High values for these tests may indicate hyperthyroidism. This can be caused by Graves' disease, thyroiditis, a goiter that contains one or more abnormal growths (nodules), or from receiving too much thyroid hormone medication.

Lower-than-normal values may mean:
Low values for these tests may indicate hypothyroidism. This can be caused by thyroid disease (such as thyroiditis), pituitary gland disease, or destruction of the thyroid gland by surgery or radiation.

T3 uptake (T3U) Normal: 24%–34%

The T3U value may be useful in interpreting the other thyroid hormone test results.

- A higher-than-normal T4 value combined with a high T3U value usually confirms the presence of hyperthyroidism, and a low thyroid-stimulating hormone (TSH) value will result.

- A higher-than-normal T4 value combined with a low T3U value often occurs during pregnancy or in women who take birth control pills. These women have normal thyroid function and will have a normal TSH value.

- A lower-than-normal T4 value combined with a low T3U value usually confirms the presence of hypothyroidism, and a high TSH value will result. Pituitary diseases would not have the same results.

- A lower-than-normal T4 value combined with an increased T3U value may indicate kidney disease or long-term (chronic) illness, and the TSH value will be normal. This can also occur normally in some healthy people.
What Affects the Test

a) Many medications may interfere with thyroid function test results, including:
Corticosteroids and hormones such as estrogen, progesterone, and birth control pills.

- Large doses of aspirin.
- Warfarin (Coumadin), diphenylhydantoin (Dilantin), carbamazepine (such as -- Tegretol), amiodarone (Cordarone), lithium (such as Eskalith, Lithonate), clofibrate (Atromid-S), phenytoin or diphenylhydantoin (Dilantin), heparin, propranolol (such as Inderal), and phenylbutazone.

b) Contrast material used for certain X-ray imaging tests may affect T4 results.

c) Pregnancy can affect thyroid function tests results.

What To Think About [list]
a) Other blood tests to help evaluate the thyroid gland include:

- A thyroid-stimulating hormone (TSH) test. TSH is a hormone released by the pituitary gland that triggers the thyroid gland to produce thyroid hormones. A TSH test measures the amount of TSH in the blood and is considered the most reliable method of detecting a thyroid problem. A TSH test is usually done before tests for T3 and T4. If the TSH test is abnormal, thyroid hormone tests may be done. For more information, see the medical test Thyroid-Stimulating Hormone.
- A thyroid antibodies test. This test measures the presence of antibodies against thyroid tissue. If antibodies are detected, an autoimmune disease (such as Hashimoto's thyroiditis or Graves' disease) that may cause the body to attack its own thyroid gland may be present.
- A thyroxine-binding globulin (TBG) test. TBG is an important protein in the blood that carries the thyroid hormones T3 and T4. TBG testing is rarely needed and is done only after more commonly used thyroid tests produce abnormal results.

b) Problems with the thyroid gland can be further investigated by a thyroid scan, ultrasound, or biopsy. For more information, see the medical test topics Thyroid Scan and Radioactive Iodine Uptake Test, Thyroid Ultrasound and Parathyroid Ultrasound, and Thyroid Biopsy.

c) 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.

d) If the results of an initial newborn thyroid screening test indicates congenital hypothyroidism, additional testing is done to determine the cause.

Author - Kattie Payne, RN, PhD
Editor - Renée Spengler, RN, BSN
Associate Editor - Daniel Greer
Primary Medical Reviewer - Caroline S. Rhoads, MD; Internal Medicine
Specialist Medical Reviewer - Alan Dalkin, MD; Endocrinology
Last Updated March 15, 2004