Triiodothyronine (T3) Tests

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. You may feel a little sting when the needle goes in or out. 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

Your provider will interpret your T3 results together with your TSH and T4 results. Ask your provider to explain what your results mean for your health.

• Normal T3 levels: Normal T3 levels alone do not rule out thyroid disease. If your TSH and free T4 are also normal, you probably do not have a thyroid condition. Limitation: Normal T3 levels alone do not rule out thyroid disease; other tests are needed for a complete picture. Next step: Discuss with your provider to confirm that your thyroid function is healthy.
• High T3 levels: High T3 levels may mean you have hyperthyroidism (too much thyroid hormone). Your provider will check your TSH and T4 to help determine the cause. Limitation: High T3 alone does not diagnose the cause; additional testing is required. Results must be interpreted with TSH and T4. Next step: Further evaluation is needed to confirm the diagnosis and identify the underlying cause.
• Low T3 levels: Low T3 levels may mean hypothyroidism (too little thyroid hormone). Your provider will check your TSH and T4 to help determine the cause. Limitation: T3 testing is not commonly used to check for hypothyroidism; other thyroid tests can detect it earlier. Results must be interpreted with TSH and T4. Next step: Further evaluation is needed to confirm the diagnosis and identify the underlying cause.

• 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.

• 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

U.S. National Library of Medicine: Triiodothyronine (T3) Tests