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Free and Bound Triiodothyronine (Blood)

Does this test have other names?

Triiodothyronine test, T3 test, thyroid function test

What is this 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:

  • Breathing

  • Heart function

  • Nervous system function

  • Body temperature

  • Cholesterol level

  • Metabolism that affects your weight

  • Brain development

  • Moisture in the skin

  • Menstruation

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.

Why do I need this test?

You may need this test if you have symptoms of hyperthyroidism. Symptoms include:

  • Sensitivity to heat

  • Weight loss

  • Irritability or nervousness

  • Fatigue or muscle weakness

  • Difficulty sleeping

  • Abnormally large thyroid gland, called a goiter

  • Very fast, irregular heartbeat

  • Diarrhea

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.

What other tests might I have along with this test?

Your doctor may also order these tests:

  • T4 test

  • Thyroid-stimulating hormone, or TSH, test

  • Thyroid scan

  • 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 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.

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 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.

How do I get ready for this test?

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.