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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. 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.footnote 1
Thyroid hormone blood tests include:
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 (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.
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.
Thyroid hormone tests are done to:
If you are taking thyroid medicines, tell your doctor when you took your last dose. You may need to stop taking thyroid medicines for a short time before having this test.
A health professional uses a needle to take a blood sample, usually from the arm.
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.
When a blood sample is taken, you may feel nothing at all from the needle. Or you might 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 having a problem from this test. When a blood sample is taken, a small bruise may form at the site.
There is very little risk of a problem from a heel stick. Your baby may get a small bruise at the puncture site.
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) 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 thyroid hormone levels (hyperthyroidism) may be caused by:
Low thyroid hormone levels (hypothyroidism) may be caused by:
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:
July 28, 2021
Author: Healthwise StaffMedical Review: Kathleen Romito MD - Family MedicineE. Gregory Thompson MD - Internal MedicineAdam Husney MD - Family Medicine
Current as of: July 28, 2021
Kathleen Romito MD - Family Medicine & E. Gregory Thompson MD - Internal Medicine & Adam Husney MD - Family Medicine
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