Thyroxine (T4) Test

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, 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.

A health care professional will take a blood sample from a vein in your arm using a small needle. The procedure 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 T4 test results are usually reported as free T4 (if a free T4 test was done) or as free T4 index (FTI) (if a total T4 test was done). To fully understand your results, your provider will compare them with other thyroid tests. The following general patterns may apply.

• Lower than normal T4: May be a sign of hypothyroidism, Hashimoto's disease, certain stages of thyroiditis, a pituitary problem (uncommon), iodine deficiency or excess, or congenital hypothyroidism. Low T4 can also occur after treatment for hyperthyroidism or thyroid cancer. Limitation: Abnormal T4 levels do not always mean you have a medical condition that needs treatment. Many factors (e.g., severe illness, certain medicines) can affect T4 levels. Next step: Your provider will likely order additional tests to find the cause and may discuss treatment options if needed.
• Higher than normal T4: May be a sign of hyperthyroidism, Graves' disease, certain stages of thyroiditis, toxic goiter, toxic thyroid nodule, too much iodine, a noncancerous pituitary tumor, or taking too much thyroid hormone medicine. Limitation: Abnormal T4 levels do not always mean you have a medical condition that needs treatment. Many factors (e.g., severe illness, certain medicines) can affect T4 levels. Next step: Your provider will likely order additional tests to find the cause and may discuss treatment options if needed.
• Normal T4: Your T4 level is within the expected range. This suggests your thyroid function is normal when considered with other thyroid tests. Limitation: A normal T4 result does not rule out all thyroid conditions. Your provider will interpret this result along with your symptoms and other tests. Next step: No immediate action is needed. Continue routine follow-up with your healthcare provider as recommended.

• 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.

• 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

U.S. National Library of Medicine: Thyroxine (T4) Test