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Thyroid Hormone Tests

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Test Overview

Thyroid hormone tests are blood tests that check how well the thyroidthyroid 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 hormoneshormones: 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 disabilityIntellectual disability may occur if a baby's thyroid gland does not produce enough thyroid hormone (congenital hypothyroidismhypothyroidism). Older children also need thyroid hormones to grow and develop normally, and adults need the hormones to regulate the way the body uses energy (metabolismmetabolism). The United States Preventive Services Task ForceUnited States Preventive Services Task Force recommends that all newborns be tested for congenital hypothyroidism.^{footnote 1footnote 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. 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 hyperthyroidismhyperthyroidism.
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.

Learn more

Thyroid-Stimulating Hormone (TSH) TestThyroid-Stimulating Hormone (TSH) Test

How To Prepare

How It Is Done

Blood test

A health professional uses a needle to take a blood sample, usually from the arm.

Heel stick

A heel stick is used to get a blood sample from a baby. The baby's heel is poked, and several drops of blood are collected. The baby may have a tiny bruise where the heel was poked.

How It Feels

Blood test

When a blood sample is taken, you may feel nothing at all from the needle. Or you might 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.

Risks

Blood test

There is very little chance of having a problem from this test. When a blood sample is taken, a small bruise may form at the site.

Heel stick

There is very little risk of a problem from a heel stick. Your baby may get a small bruise at the puncture site.

Results

Normal

Each lab has a different range for what's normal. Your lab report should show the range that your lab uses for each test. The normal range is just a guide. Your doctor will also look at your results based on your age, health, and other factors. A value that isn't in the normal range may still be normal for you.

Results are usually available within a few days.

Labs generally measure free T4 (FT4) levels, but they 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)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 past health.

High values

High thyroid hormone levels (hyperthyroidismhyperthyroidism) may be caused by:

Diseases of the thyroid gland, such as Graves' diseaseGraves' disease, thyroiditisthyroiditis, or a goitergoiter that contains one or more abnormal growths (nodules).
Taking too much thyroid medicine.

Low values

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

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

References

Citations

U.S. Preventive Services Task Force (2008). Screening for congenital hypothyroidism: Reaffirmation recommendation statement. Available online: http://www.uspreventiveservicestaskforce.org/uspstf08/conhypo/conhyprs.htm.

Current as of: April 30, 2024

Author: Ignite Healthwise, LLC StaffIgnite Healthwise, LLC Staff

Clinical Review BoardClinical Review Board
All Ignite Healthwise, LLC education is reviewed by a team that includes physicians, nurses, advanced practitioners, registered dieticians, and other healthcare professionals.

This information does not replace the advice of a doctor. Ignite Healthwise, LLC, disclaims any warranty or liability for your use of this information. Your use of this information means that you agree to the Terms of UseTerms of Use. Learn how we develop our contenthow we develop our content.

To learn more about Ignite Healthwise, LLC, visit webmdignite.comwebmdignite.com.

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