Triiodothyronine test, T3 test, thyroid function test
This test measures the level of triiodothyronine (T3) in your blood. T3 is a type of hormone made by your thyroid gland.
T3 and other thyroid hormones 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:
Nervous system function
Metabolism that affects your weight
Moisture in the skin
The T3 test is used to help diagnose thyroid problems, especially hyperthyroidism. In hyperthyroidism, the thyroid gland produces 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 T3 test measures only the amount of free T3. The total T3 test measures both free and bound T3 in your blood.
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, called a 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 B12
Have primary adrenal insufficiency, a hormone disorder
Eat a lot of foods rich in iodine
Take medication 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.
Your doctor may also order these tests:
Thyroid-stimulating hormone, or TSH, test
Radioactive iodine uptake test
Thyroid-stimulating immunoglobulin, or TSI, test
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 health care 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.
The test requires a blood sample, which is drawn through a needle from a vein in your arm.
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.
Taking thyroid hormone medications or certain other specific medications can affect your test results. Eating a diet high in iodine-rich foods, such as kelp, may also affect your test results.
You don't need to prepare for this test. But be sure your doctor 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.