Triiodothyronine (T3) Tests
Reviewed
A T3 test measures the level of triiodothyronine in your blood to help diagnose thyroid disease, pituitary disorders, or monitor thyroid treatment. It is usually interpreted alongside TSH and T4 tests.
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.
Understanding this test
Use the reference interval and units printed by the laboratory that performed the test. A result outside that interval is not a diagnosis by itself, and a result inside it does not rule out every condition. Your symptoms, history, medicines, timing, and related tests can change what the result means.
- 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
Your provider will explain your results and recommend any necessary follow-up tests or treatments.
Evidence source
U.S. National Library of Medicine: Triiodothyronine (T3) Tests