Triiodothyronine (T3) Tests
Reviewed
Also called: Total T3, Free T3, FT3, TT3
A triiodothyronine (T3) test measures the level of T3 in a sample of your blood. It's used to help diagnose thyroid disease.
Specimen: Blood
Why this test may be ordered
- Symptoms of hyperthyroidism (anxiety, weight loss, shaky hands, heat sensitivity, irregular heartbeat)
- Abnormal TSH or T4 results
- Monitoring thyroid hormone therapy
Preparation and collection
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.
A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. This usually takes less than five minutes.
There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.
Understanding results
To understand the results of your T3 test, your provider will usually compare them with the results of your TSH and T4 tests. Each combination suggests a different thyroid condition.
- Normal T3 levels: But if your TSH and free T4 levels are also normal, you probably don't have a thyroid condition. Limitation: Normal T3 levels (free or total) alone do not rule out thyroid disease. Next step: Ask your provider to explain what your test results say about your health.
- High T3 levels: High T3 levels (free or total) may mean you have hyperthyroidism (too much thyroid hormone). Limitation: Your provider will look for different combinations of high, low, and normal results. Next step: If you have questions about your results, talk with your provider.
- Low T3 levels: Low T3 levels (free or total) may mean hypothyroidism (too little thyroid hormone). Limitation: Certain medicines and supplements can affect your test results. Next step: To understand the results of your T3 test, your provider will usually compare them with the results of your TSH and T4 tests.
- Normal T3 levels alone do not rule out thyroid disease.
- Results must be interpreted together with TSH and T4 tests.
- Certain medicines and supplements can affect T3 levels.
What to do next
Ask your provider to explain what your test results say about your health. If you have questions about your results, talk with your provider.
- What do my T3, TSH, and T4 results together indicate about my thyroid function?
- Do I need any additional tests?
- Should I adjust any medications or supplements before my next test?
Evidence source
Source: U.S. National Library of Medicine (MedlinePlus). Practices and reference ranges may differ in your country.
U.S. National Library of Medicine: Triiodothyronine (T3) Tests