What Is a T4 Test?

What Is Free T4 and Bound T4?

The T4 test measures thyroxine in your blood to show how well your thyroid is working.
The T4 test measures thyroxine in your blood to show how well your thyroid is working.

T4 stands for thyroxine, one of two hormones your thyroid gland makes. The thyroid is a butterfly-shaped gland in your neck that helps control your body's growth and energy use. The T4 test measures the amount of T4 in your blood to show whether your thyroid works as well as it should. 

You have two kinds of T4 hormone:

  • Free T4 travels around your bloodstream on its own and goes wherever you need it.
  • Bound T4 attaches to proteins to travel through your blood.

A total T4 test measures both free and bound T4. A free T4 test measures only thyroxine that's not bound to proteins. Many doctors today use the free T4 test because it's more accurate than the test for bound T4.

The T4 test can also detect problems with your pituitary gland. This gland releases TSH (thyroid-stimulating hormone) that directs the thyroid to release its hormones. 

The information that follows will help you understand why your doctor has asked for a T4 test. It will also help you know what the results might mean in terms of your own health.

Why You Might Have a T4 Test

 Hypothyroidism and Hyperthyroidism Symptoms: Swollen Neck
Hypothyroidism and Hyperthyroidism Symptoms: Swollen Neck

Your doctor may order a T4 test if: 

  • You have symptoms of a thyroid problem.
  • Your thyroid gland is swollen.
  • You had an abnormal result on a TSH test.

Or you might have this test if your doctor has already diagnosed you with a thyroid disease and wants to see how well your treatment is working.

The T4 test helps your doctor diagnose hypothyroidism and hyperthyroidism.

Hypothyroidism is an underactive thyroid gland. That means your thyroid produces too little of its hormones, which slows your body down. Symptoms of hypothyroidism include:

Hyperthyroidism is an overactive thyroid gland. Your thyroid produces too much hormone and your body speeds up. Symptoms of hyperthyroidism include:

The T4 Blood Test Procedure

How to Prepare

You don't need to do anything special before this test. But, do tell your doctor about any medicines or supplements you take -- even ones you bought without a prescription. Some drugs, including seizure medicines, heart drugs, and birth control pills, can affect test results. The supplement biotin can cause a false-positive result, where your test results are abnormal but you don't really have a thyroid problem.

Also tell your doctor if you've been sick. Some infections can alter your test results. You may need to wait until you feel better to have this test. Chronic diseases like kidney failure and cirrhosis may also affect T4 test results.

What to Expect

The T4 test is a blood test. Your health care provider will use a needle to draw blood from a vein in your arm. You may feel a slight prick as the needle goes in. Afterward, the provider will remove the needle and place a bandage over the site.

What Your T4 Blood Test Results Mean

You should get the results of your test within a day or so. Results can vary based on your age, gender, and the method your lab uses to read the result.

Normal Range

The normal range for free T4 in adults is 4.6 to 11.2 micrograms per deciliter (mcg/dL). 

A normal T4 test result means your thyroid gland works like it should.

High or Low T4

A high T4 result could mean you have hyperthyroidism. A low T4 test result is a sign of hypothyroidism. 

Your hypothalamus and pituitary glands control the release of T4. An abnormal result on the T4 test could also be a sign of a problem with one of these glands.

Other Thyroid Tests You Might Need

To help your doctor diagnose the exact problem you have, you may also need tests of the following:

  • T3 -- the other hormone your thyroid gland makes, works with T4 to control your body's energy use
  • Thyroid-stimulating hormone (TSH) -- a hormone your pituitary gland makes, tells your thyroid to make T3 and T4
  • Thyroid antibodies -- proteins your immune system makes if it mistakenly sees your thyroid gland as foreign, like in Hashimoto's thyroiditis or Graves' disease

T4 Blood Test Side Effects

The T4 test is just a basic blood test. Any side effects it might cause are minor, including:

After Your T4 Lab Test

Your test results will help your doctor decide what you should do next. If your results are normal, you may not need to do anything else. An abnormal result on this and other thyroid tests may mean you need treatment for a thyroid condition.

(c)2019 WebMD, LLC. All rights reserved.

American Thyroid Association: "Hypothyroidism (Underactive)," "Thyroid Function Tests."

KidsHealth.org: "Blood Test: T4 (Thyroxine)."

Lab Tests Online: "T3, Free and Total," "T4, Free," "Thyroid antibodies," "Thyroid-stimulating Hormone (TSH)."

Mayo Clinic: "Hyperthyroidism (overactive thyroid)." "Hypothyroidism (underactive thyroid)."

University of Rochester Medical Center: "Free T4." "Free and Bound T4."