Thyroxine (T4) Test
Reviewed
A T4 test measures the level of thyroxine (T4) in your blood. T4 is the main hormone made by your thyroid gland. The test helps evaluate thyroid function and is usually done together with a TSH test.
Preparation and collection
In most cases, no special preparation is needed. Tell your provider about any medicines or supplements you take, but do not stop any medicines without talking to your provider. If additional tests are ordered, you may need to fast (not eat or drink) for several hours before the test.
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.
- A T4 test alone cannot provide enough information to diagnose thyroid problems; it is usually done with a TSH test.
- Many factors can affect T4 levels, including severe illness and certain medicines (e.g., birth control pills, steroids).
- Abnormal T4 levels do not always mean you have a medical condition that needs treatment.
- Results may vary depending on whether free T4 or total T4 (FTI) was measured.
What to do next
Discuss your T4 results with your healthcare provider, who will interpret them along with other thyroid tests and your symptoms. If your results are abnormal, you may need additional testing to find the cause. Do not stop or change any medicines without talking to your provider.