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  • Also Known As:
  • Thyroxine
  • Free T4
  • Total T4
  • T4 Index
  • Thyroxine Test by Equilibrium Dialysis
  • Formal Name:
  • Free Thyroxine
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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 T4, including free T4 and T4 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. A high level of T4 may be a sign of an overactive thyroid, called hyperthyroidism. A T4 test may be ordered 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 in patients 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 or thyroxine 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 for 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 T4 testing?

There are a number of 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 a patient has symptoms that may be related to a thyroid disorder. Underactive thyroid (hypothyroidism) and overactive thyroid (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 United States. Symptoms of hypothyroidism 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 a patient’s 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 to get tested

T4 testing is usually performed at a doctor’s office or other medical setting like a hospital or lab. T4 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 thyroid panels. Most include TSH, T3, and free T4. Some also include thyroid antibodies.

The 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 will be able to access your test results via a secure online platform. If you decide to purchase an at-home thyroid panel, it’s important to discuss your results with your doctor, who can help you interpret the results. If 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.

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Taking the 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. For screening tests in newborns, a blood sample is typically taken from the baby’s heel.

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) to be within the normal range for free T4.

Your results may be described as free T4 or free thyroxine index (FTI). FTI means 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 is testing 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:

Thyroid-stimulating Hormone (TSH) Free T4 Thyroid Function
Normal Normal Normal thyroid function
High Normal Subclinical hypothyroidism
High High Hyperthyroidism related to pituitary gland
High Low Hypothyroidism related to the thyroid gland
Low Normal Subclinical hyperthyroidism
Low High Hyperthyroidism related to the thyroid gland
Low Low Hypothyroidism related to pituitary gland


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 in people being treated 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.

Are test results accurate?

While no test is perfect, free T4 test results are generally accurate. However, there are things that may interfere with the results. Certain drugs or supplements may affect results and should be discussed in advance with your doctor. Measurement of free T4 during pregnancy may require special considerations due to chemical changes in the body. Total T4 and free T4 levels may also be misleading in patients who are very ill or malnourished because of low levels of the proteins that normally bind to T4.

Do I need follow-up tests?

Sometimes you may need additional testing after an abnormal T4 test result. The following circumstances may prompt follow-up testing:

  • Patients with untreated subclinical hypothyroidism may have follow-up testing with TSH and free T4 six months after diagnosis and then annually.
  • Patients with untreated subclinical hyperthyroidism may have follow-up testing with TSH, free T4, and T3 every six months.
  • Newborns screened with T4 and TSH for congenital hypothyroidism who have an abnormal result will be re-tested and further evaluated by their physician.
  • Patients diagnosed with thyroid disorders typically have periodic thyroid function tests to ensure that their conditions are stable and to monitor the effectiveness of treatment.

Questions for your doctor about test results

  • Is my T4 test result normal?
  • Are you able to 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?

Related Tests

How is free T4 different from total T4?

Most of the T4 in the body is bound to proteins, with less than 1% typically being “free” or available for immediate use by the body. Protein-bound T4 and free T4 are measured together in a test called total T4.

The total T4 test is less reliable than free T4 because a number of factors may change the levels of protein in the body binding to T4. These protein levels—and therefore your total T4 level—can be affected by pregnancy, oral contraceptive use, severe illness, or the use of medicines called corticosteroids. Many healthcare providers prefer to use the free T4 test because it is not affected by changes in the levels of binding proteins and is therefore considered more accurate.


A.D.A.M. Medical Encyclopedia. Venipuncture. Updated April 26, 2019.  Accessed July 3, 2021. https://medlineplus.gov/ency/article/003423.htm

A.D.A.M. Medical Encyclopedia. Free T4 test. Updated January 26, 2020. Accessed July 3, 2021. https://medlineplus.gov/ency/article/003517.htm

American Board of Internal Medicine. ABIM laboratory test reference ranges. Updated January 2021. Accessed July 5, 2021. https://www.abim.org/Media/bfijryql/laboratory-reference-ranges.pdf

American Thyroid Association. Thyroid function tests. Date unknown. Accessed July 5, 2021. https://www.thyroid.org/thyroid-function-tests/

Hammami MB. Thyroxine. Staros EB, ed. Medscape. Updated November 29, 2018. Accessed July 3, 2021. https://emedicine.medscape.com/article/2089576-overview

National Institute of Diabetes and Digestive and Kidney Diseases. Hyperthyroidism (overactive thyroid). Updated August 2016. Accessed July 5, 2021. https://www.niddk.nih.gov/health-information/endocrine-diseases/hyperthyroidism

National Institute of Diabetes and Digestive and Kidney Diseases. Thyroid tests. Updated May 2017. Accessed July 5, 2021. https://www.niddk.nih.gov/health-information/diagnostic-tests/thyroid

National Institute of Diabetes and Digestive and Kidney Diseases. Hypothyroidism (underactive thyroid). Updated March 2021. Accessed July 5, 2021. https://www.niddk.nih.gov/health-information/endocrine-diseases/hypothyroidism

National Heart, Lung, and Blood Institute. Blood tests. Date unknown. Accessed July 5, 2021. https://www.nhlbi.nih.gov/health-topics/blood-tests

LaFranchi S. Clinical features and detection of congenital hypothyroidism. In: Geffner ME, ed. UpToDate. Updated April 9, 2021. Accessed July 5, 2021. https://www.uptodate.com/contents/clinical-features-and-detection-of-congenital-hypothyroidism

Ross DS. Laboratory assessment of thyroid function. In: Cooper DS, ed. UpToDate. Updated December 11, 2019. Accessed July 3, 2021. https://www.uptodate.com/contents/laboratory-assessment-of-thyroid-function

Ross DS. Subclinical hyperthyroidism in nonpregnant adults. In: Cooper DS, ed. UpToDate. Updated March 12, 2021. Accessed July 6, 2021. https://www.uptodate.com/contents/subclinical-hyperthyroidism-in-nonpregnant-adults

Ross DS. Thyroid hormone synthesis and physiology. In: Cooper DS, ed. UpToDate. Updated March 29, 2021. Accessed July 3, 2021. https://www.uptodate.com/contents/thyroid-hormone-synthesis-and-physiology

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