Test Overview
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.
1
Thyroid hormone blood tests include:
-
Total thyroxine (T4). Most of the thyroxine (T4)
in the 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.
-
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. For more information, see the topic
Thyroid-Stimulating Hormone (TSH). 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
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
intellectual disability, if it is not treated soon after
birth.
How To Prepare
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?)
.
How It Is Done
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.
How It Feels
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.
Risks
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, 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.
Results
Thyroid hormone tests are blood tests
that check how well the
thyroid gland is working.
Normal
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.
Thyroid hormone tests
2
| 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 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.
What Affects the Test
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.
- Antiseizure medicines such as phenytoin or
carbamazepine.
- Heart medicines such as amiodarone or
propranolol.
- Lithium.
- Having recently had an X-ray test that uses
contrast material
.
- Being pregnant.
What To Think About
- 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.
- 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 topic
Thyroid-Stimulating Hormone.
-
Thyroid antibodies 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:
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.
-
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.
Credits
|
By
| Healthwise Staff |
|
Primary Medical Reviewer
| Kathleen Romito, MD - Family Medicine |
|
Specialist Medical Reviewer
| Matthew I. Kim, MD - Endocrinology |
|
Last Revised
| August 7, 2012 |
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.
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