Triiodothyronine (T3) Tests Test
A T3 test measures the level of triiodothyronine in your blood to help diagnose thyroid disease, pituitary disorders, or monitor thyroid treatment.
What this test measures
A triiodothyronine (T3) test measures the level of T3 in a sample of your blood. There are two forms: free T3 (active) and bound T3 (attached to proteins). Total T3 measures both together; free T3 measures only the active form.
What your results mean
Reference ranges vary by laboratory and by your age and sex. MedlinePlus does not publish a single normal range for this test — always read your result against the range printed on your own lab report and discuss it with your provider.
Higher-than-normal results
High T3 levels may mean you have hyperthyroidism (too much thyroid hormone). Depending on TSH and T4 levels, it may indicate a thyroid gland problem or a pituitary gland condition.
Lower-than-normal results
Low T3 levels may mean hypothyroidism (too little thyroid hormone). Depending on TSH and T4 levels, it may indicate a thyroid gland problem or a pituitary gland condition.
Frequently asked questions
What is a T3 test?
A T3 test measures the level of triiodothyronine (T3) in your blood. It helps diagnose thyroid disease and pituitary disorders.
What is the difference between free T3 and total T3?
Free T3 is the active form that enters tissues. Bound T3 is attached to proteins. Total T3 measures both bound and free T3 together.
Why is a T3 test done?
It is used to diagnose hyperthyroidism, pituitary disorders, and to monitor treatment for thyroid disease.
How are T3 test results interpreted?
Results are compared with TSH and T4 tests. High T3 may mean hyperthyroidism; low T3 may mean hypothyroidism. Your provider will explain what your results mean.
Can a normal T3 level rule out thyroid disease?
No, normal T3 levels alone do not rule out thyroid disease. But if TSH and free T4 are also normal, you probably don't have a thyroid condition.
Related lab tests
Sources
- MedlinePlus: Triiodothyronine (T3) Tests — NIH MedlinePlus
Last updated . Information is aggregated from official public sources and is not a substitute for professional medical care.