What’s a T4 test?

Your thyroid produces a hormone called thyroxine, which is known as T4. This hormone plays a role in several of your body’s functions, including growth and metabolism.

Some of your T4 exists as free T4. This means it hasn’t bonded to protein in your blood. This is the type available for use by your body and tissues. However, most of the T4 in your bloodstream is bonded to protein.

Because T4 exists in two forms in your body, there are two kinds of T4 tests: a total T4 test and a free T4 test.

A total T4 test measures the T4 that’s bonded to protein along with any free T4. A free T4 test measures only the free T4 in your blood. Because free T4 is what’s available to your body for use, a free T4 test is often preferred over a total T4 test.

Your doctor may order a T4 test if a thyroid-stimulating hormone (TSH) test has come back with abnormal results. A T4 test will help your doctor determine what type of problem is affecting your thyroid.

Some disorders that affect thyroid function include:

Your doctor may suspect that one of these conditions is present if you have symptoms such as:

You may also experience more general symptoms, such as:

Sometimes, your doctor may also order other thyroid tests (like T3 or TSH) after you have a T4 test.

TSH, or thyroid-stimulating hormone, comes from the pituitary gland. It stimulates your thyroid to release both T3 and T4. Performing one or both of these other tests may help your doctor gain a better understanding of your thyroid problem.

In some cases, your doctor may perform one or more of these tests to help assess whether a known thyroid issue is improving.

A number of drugs can interfere with your T4 levels, especially total T4, so it’s important to tell your doctor what medications you’re taking. You may need to temporarily stop taking certain drugs before the test to help ensure accurate results.

It’s also important to let your doctor know if you may be pregnant.

Medications that can affect your T4 levels include:

  • drugs that contain hormones, such as androgen, estrogen, and birth control pills
  • drugs designed to affect your thyroid or treat thyroid conditions
  • some drugs designed to treat cancer
  • steroids

These aren’t the only drugs that can affect your results. Make sure to tell your doctor about all medications, as well as any herbal supplements, you use.

A healthcare provider will collect your blood into a tube or vial and send it to a lab for testing.

Typical results for the total T4 test in adults generally range from 5.0 to 12.0 micrograms per deciliter (μg/dL). Results for children vary based on age. Talk to your doctor about the normal ranges expected for your child. There may also be some variation between labs.

Typical results in adults for the free T4 test generally range from 0.8 to 1.8 nanograms per deciliter (ng/dL). Like total T4 in adults, free T4 also varies in children according to age.

As with any test result, if it falls outside the expected range, talk to your doctor about what the results may mean in regards to your own personal health condition.

Because T4 isn’t the only hormone involved in thyroid function, a normal result on this test could still mean there’s a thyroid problem.

For example, your T4 results could fall into a normal range, but your T3 results could be abnormal. This can be particularly true in cases of an overactive thyroid.

You can also order an at-home thyroid test to measure T4 levels.

An abnormal result on the T4 test alone may not give your doctor enough information to fully understand or diagnose your condition. They may also need to consider the results of T3 and TSH levels for a more complete picture.

Pregnancy can also affect your T4 levels. If your T4 levels are abnormal but you’re pregnant, your doctor may order further testing.

Abnormally high test results

Elevated T4 levels may indicate hyperthyroidism. They may also indicate other thyroid problems, such as thyroiditis or toxic multinodular goiter.

Other reasons for abnormal results may include:

  • high levels of protein in the blood
  • too much iodine
  • too much thyroid replacement medication
  • trophoblastic disease, a group of rare pregnancy related tumors
  • germ cell tumors

Too much iodine can raise your T4 levels. Because X-ray dyes can include iodine, a recent X-ray involving dye may also raise your T4 test results.

Abnormally low test results

Abnormally low levels of T4 may indicate:

  • dietary issues, such as fasting, malnutrition, or an iodine deficiency
  • medications that affect protein levels
  • hypothyroidism
  • illness
  • a pituitary problem

A T4 test has no specific risks. Risks include those present whenever you have your blood drawn.

In rare cases, you may experience a complication, such as:

More commonly, you may feel pain or discomfort during the blood draw. You may also bleed slightly after the needle is removed. You may develop a small bruise around the puncture site.

A T4 test is a blood test that helps your doctor identify problems with your thyroid. It’s a low risk procedure that’s often used with other blood tests, including a T3 test and a TSH test.

If you’re preparing for a T4 test, make sure to tell your doctor about all medications and supplements you’re taking, any known thyroid conditions, and if you’re pregnant. This will ensure the most accurate interpretation of the test results.