Thyroid Hormone Tests
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.
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.
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
- 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
- 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
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.
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?).