Test Quick Guide

T4, also called thyroxine, is the main form of thyroid hormone made by the thyroid gland. Most T4 is bound to proteins, while a small proportion is unbound or free. Free T4 testing measures T4 that circulates through the blood and is available to enter body tissues and act upon them. Total T4 testing measures the total amount of the hormone, including free T4 and the part that is bound to proteins.

T4 is measured by taking a blood sample. A low T4 test result may indicate an underactive thyroid gland or problems with its stimulation by the pituitary gland. And a high level of T4 may be a sign of an overactive thyroid, called hyperthyroidism. Your doctor may order a T4 test if thyroid problems are suspected or to follow up after an abnormal thyroid-stimulating hormone (TSH) test result.

About the Test

Purpose of the test

T4 testing helps your doctor evaluate how well your thyroid is working. This test may be ordered to:

  • Follow up on an abnormal TSH test result
  • Diagnose hyperthyroidism, a condition involving an overactive thyroid gland
  • Diagnose hypothyroidism, a condition in which the thyroid gland is underactive
  • Monitor T4 levels if you are on thyroid replacement therapy
  • Screen for an underactive thyroid gland in newborn babies
  • Evaluate other disorders, such as goiters, thyroid nodules, and problems with the pituitary or hypothalamus gland

What does the test measure?

T4 is the major hormone made in your thyroid gland. The name T4 derives from the fact that the thyroxine molecule has four iodine atoms attached to it.

Thyroid hormones are critical to brain development during infancy and many essential body processes during adulthood. Usually, your body has large stores of T4 circulating in your blood. However, most of your circulating T4 is bound to proteins and is not available for immediate use by the body.

Free T4 testing measures only the active form of thyroid hormone, which is unbound and can directly enter cells and affect them. Total T4 testing measures both bound and unbound T4 in the blood.

When should I get this test?

There are many different reasons why a doctor might order a T4 test. Often it is ordered to follow up on an abnormal TSH test result.

It may also be used when you have symptoms that may be related to a thyroid disorder. Hypothyroidism and hyperthyroidism are both common diseases. If you are having symptoms of these disorders, your doctor may order a T4 test in combination with other thyroid function tests, such as TSH and T3. This series of tests is also called a thyroid panel.

Hypothyroidism affects nearly 5% of adolescents and adults in the U.S. Symptoms may include:

  • Fatigue
  • Weight gain
  • Feeling cold
  • Painful joints and muscles
  • Dry skin
  • Thin and/or dry hair
  • Slow heart rate
  • Constipation
  • Depression
  • Irregular menstrual periods
  • Fertility problems

Another common thyroid disorder is hyperthyroidism, which affects slightly more than 1% of people in the U.S.

  • Weight loss
  • Anxiety
  • Insomnia
  • Irritability
  • Difficulty tolerating heat
  • Mood swings
  • Rapid or irregular heartbeat
  • Loose, frequent bowel movements

There are additional reasons why a doctor might check your T4, such as monitoring the effectiveness of treatment with replacement thyroid hormones, screening newborns for congenital hypothyroidism, and to evaluate possible problems with the hypothalamus or pituitary gland.

Finding a T4 Test

How can I get a T4 test?

T4 testing is usually performed at a doctor’s office or another medical setting like a hospital or lab. The tests are normally ordered by a doctor but may be available without orders from your doctor at a walk-in lab.

Can I take the test at home?

It is possible to take a free T4 test at home as part of a thyroid panel test. Companies vary slightly as to what thyroid function tests are included in their panels. Most include TSH, T3, and free T4. Some also include thyroid antibodies.

Commercially available kits for testing thyroid hormones enable you to provide a blood sample using a finger prick. The sample is then mailed to a lab for testing.

After your sample is analyzed, you can access your test results via a secure online platform. If you purchase an at-home thyroid panel, it’s important to discuss your results with your doctor, who can help you interpret them. Should you receive an abnormal result on an at-home test, your doctor may recommend retesting your T4 level.

How much does the test cost?

The cost of a T4 test will vary depending on factors such as where the test is done and whether you have health insurance. When ordered by a doctor, insurance typically covers the test, although you may have to pay a copay or deductible. Your doctor’s office, lab, and health plan can provide information about any out-of-pocket costs that may be your responsibility.

Taking a T4 Test

The T4 test requires a blood sample, which is usually taken from your arm in a doctor’s office, health clinic, hospital, or lab. A blood sample is typically taken from the baby’s heel for screening tests in newborns.

Before the test

Usually, no special preparation is required for a T4 test. Your doctor will tell you if you need to stop taking any medications or supplements before the test. Biotin (B7) supplements may need to be discontinued briefly to prevent them from interfering with your test results.

During the test

A blood sample is taken from a vein in your arm. The person taking the sample may tie a band around your upper arm and will clean the area where the needle will be inserted into your skin. A small amount of blood is drawn into a tube. You may feel a slight sting when the needle enters your skin.

The process of taking a blood sample usually takes less than three minutes.

After the test

At a doctor’s office or lab, you will be asked to apply gentle pressure to the site with a bandage or a piece of gauze after the needle is withdrawn. This will help stop bleeding and may prevent bruising. Next, the site will be bandaged. You may resume your normal activities following the test.

A blood draw is a very low-risk procedure. You may have slight bruising at the site where the blood sample was taken.

T4 Test Results

Receiving test results

The doctor who ordered your T4 test may share the results with you, or you may be able to access them through an online patient portal. T4 results are usually available within a few business days.

Interpreting test results

Your T4 level will be reported either as nanograms per deciliter (ng/dL) or picomoles per liter (pmol/L). The reference range, or what is considered normal for T4, may vary slightly from laboratory to laboratory. For example, the American Board of Internal Medicine uses a reference range for free T4 of 0.8–1.8 ng/dL, while the National Library of Medicine’s A.D.A.M. Medical Encyclopedia considers 0.9 to 2.3 ng/dL (12-30 pmol/L) within the normal range for free T4.

Your results may be described as free T4 or free thyroxine index (FTI) — when the test result was calculated using a formula that compares total T4 and some estimation of the level of thyroid hormone proteins.

If your doctor tests your free T4 to diagnose or rule out a thyroid or pituitary gland disorder, your test results will be considered along with the TSH. The table below demonstrates typical patterns found in these disorders:

Subclinical hypothyroidism and hyperthyroidism are defined by abnormal lab test results. They often cause only mild or no symptoms. Over time, some subclinical thyroid conditions may progress to outright hypothyroidism or hyperthyroidism.

Free T4 testing may be used to monitor response to treatment for thyroid disorders. If your free T4 is measured for treatment monitoring, your doctor can explain what your test results reveal about how well your treatment is working.

You may want to ask your doctor the following questions:

  • Can you make a diagnosis based on my T4 test results, along with my other thyroid function test results?
  • Do you recommend treatment or monitoring based on my T4 test results?
  • What follow-up tests will be ordered and when will they take place?

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