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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:
Thyroid hormone tests are done to:
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? ).
The health professional taking a sample of your blood will:
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.
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.
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):
11.8–22.6 micrograms per deciliter (mcg/dL) or 152–292 nanomoles per liter (nmol/L) in newborns
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):
0.7–2.0 ng/dL nanograms per deciliter (ng/dL) or 10–26 picomoles per liter (pmol/L)
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:
Low thyroid hormone levels (hypothyroidism) may be caused by:
Reasons you may not be able to have the test or why the results may not be helpful 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. Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
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.
ByHealthwise Staff Primary Medical Reviewer Kathleen Romito, MD - Family Medicine E. Gregory Thompson, MD - Internal Medicine Adam Husney, MD - Family Medicine
Current as ofMarch 15, 2018
Current as of:
March 15, 2018
Kathleen Romito, MD - Family Medicine & E. Gregory Thompson, MD - Internal Medicine & Adam Husney, MD - Family Medicine
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