Free and Bound Triiodothyronine (Blood)
Does this test have other names?
Triiodothyronine test, T3 test, thyroid function test, FT3
What is this test?
This blood test measures the level of the hormone triiodothyronine (T3) in your blood.
The hormone is made by your thyroid gland. The thyroid is a butterfly-shaped gland
near the base of your throat above your collarbones.
The thyroid gland makes T3 in response to thyroid stimulating hormone (TSH). TSH is
made by the pituitary gland in your brain.
T3 works with another thyroid hormone called T4 to help control how your body stores
and uses energy to do its work (metabolism). The thyroid hormones help control many
of your body's other processes, including:
The T3 test is used to help diagnose thyroid problems, such as hyperthyroidism. Hyperthyroidism
means the thyroid gland makes too many thyroid hormones.
T3 has 2 forms: bound and free. Bound T3 is attached to a protein and free T3 is not
attached to anything. The free T-3 test measures only the amount of free T3. The total
T3 test measures both free and bound T3 in your blood.
Why do I need this test?
You may need this test if you have symptoms of hyperthyroidism. Symptoms include:
Anxiety and mood swings
Weakness in the arms and legs
Low tolerance for heat
Unexplained weight loss
More frequent bowel movements than usual
Eye irritation or bulging eyes (these are symptoms of Graves disease, a common cause
Enlarged breasts and erectile dysfunction in men
Thinning of hair
High blood sugar
Shortness of breath
You may also need this test if you are at risk for hyperthyroidism and you:
Are older than 60 years of age
Have a thyroid problem
Have a family member with a thyroid problem
Have type 1 diabetes
Have pernicious anemia, a type of anemia caused when your body can't absorb vitamin
Have primary adrenal insufficiency, a hormone disorder
Eat a lot of foods rich in iodine
Take medicine that contains iodine
Have recently been pregnant or had a baby
You may also need this test if you have symptoms of hypothyroidism. This is when the
thyroid gland does not make enough thyroid hormones. Symptoms include:
What other tests might I have along with this test?
You may also need these tests:
What do my test results mean?
Test results may vary depending on your age, gender, health history, and other things.
Your test results may be different depending on the lab used. They may not mean you
have a problem. Ask your healthcare provider what your test results mean for you.
Results of this test are given in nanograms per deciliter (ng/dL) and picograms per
deciliter (pg/dL). A normal level of total T3 (free and bound) in adults in the blood
is 80 to 200 ng/dL. The normal level of free T3 in the blood is 260 to 480 pg/dL.
How is this test done?
The test is done with a blood sample. A needle is used to draw blood from a vein in
your arm or hand.
Does this test pose any risks?
Having a blood test with a needle carries some risks. These include bleeding, infection,
bruising, and feeling lightheaded. When the needle pricks your arm or hand, you may
feel a slight sting or pain. Afterward, the site may be sore.
What might affect my test results?
Taking thyroid hormone medicines or certain other medicines, such as estrogen or methadone,
can affect your test results. Eating a diet high in iodine-rich foods, such as seaweed,
may also affect your test results.
How do I get ready for this test?
You don't need to prepare for this test. But be sure your healthcare provider knows
about all the medicines, herbs, vitamins, and supplements you are taking. This includes
medicines that don't need a prescription and any illegal drugs you may use.