Free and Bound Triiodothyronine (Blood)
Does this test have other names?
Triiodothyronine test, T3 test, thyroid function test
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 help to control how your body stores and uses energy (metabolism). The thyroid hormones help control many of your body's other processes, including:
Nervous system function
Metabolism that affects your weight
Moisture in the skin
The T3 test is used to help diagnose thyroid problems, especially 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 of hyperthyroidism
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
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 B-12
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:
Low tolerance for cold
Slower heart rate
Shortness of breath
Loss of consciousness (rare)
What other tests might I have along with this test?
You may also need these tests:
Radioactive iodine uptake test
Thyroid-stimulating immunoglobulin, or TSI, test
What do my test results mean?
Test results may vary depending on your age, gender, health history, the method used for the test, and other things. Your test results 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). A normal level of total T3 (free and bound) in the blood is 75 to 195 ng/dL. The normal level of free T3 in the blood is 0.2 to 0.5 ng/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 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 medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illegal drugs you may use.
June 09, 2018
Laboratory assessment of thyroid function, UpToDate
Bass, Pat F. III, MD, MPH,Haldeman-Englert, Chad, MD