Thyroxine (T4) Test
Reviewed
Also called: Free Thyroxine, Free T4, Total T4 Concentration, Free T4 Index (FTI), Thyroxine Screen, Free T4 Concentration
A T4 test measures the level of T4 in a sample of your blood. Too much or too little T4 may be a sign of a thyroid problem.
Specimen: Blood
Why this test may be ordered
- To help diagnose hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid)
- To monitor treatment if you are taking thyroid hormone medicine for hypothyroidism
- To check a newborn for congenital hypothyroidism
- To gather more information about other thyroid conditions such as nodules or goiter
- To help evaluate disorders of the pituitary gland or hypothalamus
Preparation and collection
In most cases, you don't need any special preparations for a T4 blood test. Certain medicines and supplements may affect your test results, so let your provider know what you're taking, but don't stop any medicines without talking with your provider.
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
To fully understand your results, your provider will usually need to compare them with the results of other thyroid tests.
- Lower than normal T4: In general, T4 results that are lower than normal may be a sign of: Hypothyroidism Hashimoto's disease, an autoimmune disorder that causes your immune system to attack your thyroid Limitation: Low T4 levels may also be caused by treatment for hyperthyroidism or thyroid cancer, including thyroid radiation therapy, radioactive iodine, and surgery to remove all or part of your thyroid. Next step: If your T4 results are abnormal, you may need more testing to find the cause.
- Higher than normal T4: In general, T4 results that are higher than normal may be a sign of: Hyperthyroidism Graves' disease, an autoimmune disorder that causes your immune system to attack your thyroid Limitation: Many things can affect your T4 levels, including severe illness and certain medicines, such as birth control pills and steroids. Next step: If your T4 results are abnormal, you may need more testing to find the cause.
- 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
If your T4 results are abnormal, you may need more testing to find the cause. So, it's best to talk with your provider about what your results mean.
- What do my T4 results mean when combined with my TSH and other thyroid test results?
- Do I need any additional tests to confirm a diagnosis?
- How often should I have my thyroid levels checked?
- Are there any medicines or supplements I am taking that could affect my T4 levels?
- Should I be concerned about any symptoms I am having?
Evidence source
Source: U.S. National Library of Medicine (MedlinePlus). Practices and reference ranges may differ in your country.