Triiodothyronine (T3) Tests

A triiodothyronine (T3) test measures the level of T3 in a sample of your blood. It's used to help diagnose thyroid disease. Your thyroid is a small, butterfly-shaped gland at the base of your throat.

It makes two hormones, T3 and T4 (thyroxine). Hormones are chemical messengers in your bloodstream that control the actions of certain cells or organs. Thyroid hormones work together to control the way your body uses energy.

They affect your weight, heart, body temperature, muscle strength, and even your mood. In children, thyroid hormones affect growth, too. There are two forms of T3 in your blood: Free T3 is the active form that enters your tissues where it's needed.; Bound T3 is attached to certain proteins which prevent it from entering your tissues.

Most of your T3 is bound.; There are different tests for measuring T3: A total T3 test measures both bound and free T3 together. Medical experts think that this test is the more accurate way to measure T3.; A free T3 test only measures free T3.;

Why this test is done

You may need a T3 test if you have symptoms of hyperthyroidism (too much thyroid hormone). Symptoms may include:

• Anxiety, nervousness, irritability
• Weight loss, even though your appetite may increase
• Shaky hands, muscle weakness
• Sweating or being very sensitive to heat
• Irregular heartbeat (arrhythmia)
• Frequent bowel movements (poops)
• Goiter (an enlarged thyroid gland)
• Trouble sleeping and fatigue

You may also need a T3 test if:

• You take thyroid hormone medicine to treat a thyroid problem. A T3 test can check whether you're taking the right dose to keep your thyroid hormone levels in a healthy range.
• You had abnormal results on a TSH and/or T4 test. Providers usually order these tests first to look into a possible thyroid disorder. If your test results are abnormal or unclear, your provider may use a T3 test to confirm or rule out hyperthyroidism.

T3 testing isn't commonly used to help check for hypothyroidism (too little thyroid hormone). That's because other thyroid tests can diagnose hypothyroidism earlier than a T3 test.

How to prepare

You don't need any special preparations for a T3 blood test. Certain medicines and supplements can affect your test results. Your provider will let you know if you need to stop taking any medicines before your test.

But don't stop taking any medicines unless your provider tells you to.

Sample type

Blood

Other names for this test

thyroid function test; total triiodothyronine, free triiodothyronine, FT3, TT3, radioimmunoassay FT3