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 thyroid gland makes the hormones T4 (thyroxine) and T3 in response to thyroid stimulating hormone (TSH). TSH is made by the pituitary gland. The thyroid gland is a butterfly-shaped gland near the base of your throat, above your collarbone.
T3 and T4 help to control your metabolism, which is your body's process of storing and using energy. The thyroid hormones help control many of your body's other processes, including:
The T3 test is used to help diagnose thyroid problems, especially hyperthyroidism. In hyperthyroidism, the thyroid gland makes too much thyroid hormone.
T3 has two 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:
Sensitivity to heat
Irritability or nervousness
Fatigue or muscle weakness
Abnormally large thyroid gland (goiter)
Very fast, irregular heartbeat
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
If you have already been diagnosed with hyperthyroidism, you may also need this test to find out how severe your condition is.
You may also have this test if you have symptoms of hypothyroidism, or less than normal thyroid activity. Symptoms include:
What other tests might I have along with this test?
Your healthcare provider may also order these tests:
Radioactive iodine uptake test
Thyroid-stimulating immunoglobulin, or TSI, test
What do my test results mean?
Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your healthcare provider.
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 requires a blood sample, which is drawn through a needle from a vein in your arm.
Does this test pose any risks?
Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.
What might affect my test results?
Taking thyroid hormone medicines or certain other specific medicines can affect your test results. Eating a diet high in iodine-rich foods, such as kelp, 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 illicit drugs you may use.
- Hanrahan, John, MD
- Sather, Rita, RN