• Also Known As:
  • Thyroxine
  • Formal Name:
  • Free Thyroxine
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At a Glance

Why Get Tested?

To help evaluate thyroid gland function; to help diagnose thyroid disease; to monitor effectiveness of thyroid treatment; sometimes a free T4 is used to help diagnose congenital hypothyroidism in newborns

When To Get Tested?

When you have signs and symptoms of thyroid disease and/or an enlarged thyroid (goiter) or when you have a thyroid nodule (a small lump on the thyroid gland that may be solid or fluid-filled cyst), usually after an abnormal result on a TSH test; when you are being treated for a thyroid disorder

Sample Required?

A blood sample drawn from a vein in your arm or from pricking the heel of an infant

Test Preparation Needed?

None needed; however, certain medications, multivitamins and supplements can interfere with the free T4 test, so tell your healthcare provider about any prescribed or over-the-counter drugs and/or supplements that you are taking. If you take thyroid hormone as treatment for thyroid disease, it is recommended that your blood sample be drawn before you take your dose for that day. Acute illness may affect thyroid testing results. It is generally recommended that thyroid testing be avoided in hospitalized patients or deferred until after a person has recovered from an acute illness.

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You may be able to find your test results on your laboratory’s website or patient portal. However, you are currently at Lab Tests Online. You may have been directed here by your lab’s website in order to provide you with background information about the test(s) you had performed. You will need to return to your lab’s website or portal, or contact your healthcare practitioner in order to obtain your test results.

Lab Tests Online is an award-winning patient education website offering information on laboratory tests. The content on the site, which has been reviewed by laboratory scientists and other medical professionals, provides general explanations of what results might mean for each test listed on the site, such as what a high or low value might suggest to your healthcare practitioner about your health or medical condition.

The reference ranges for your tests can be found on your laboratory report. They are typically found to the right of your results.

If you do not have your lab report, consult your healthcare provider or the laboratory that performed the test(s) to obtain the reference range.

Laboratory test results are not meaningful by themselves. Their meaning comes from comparison to reference ranges. Reference ranges are the values expected for a healthy person. They are sometimes called “normal” values. By comparing your test results with reference values, you and your healthcare provider can see if any of your test results fall outside the range of expected values. Values that are outside expected ranges can provide clues to help identify possible conditions or diseases.

While accuracy of laboratory testing has significantly evolved over the past few decades, some lab-to-lab variability can occur due to differences in testing equipment, chemical reagents, and techniques. This is a reason why so few reference ranges are provided on this site. It is important to know that you must use the range supplied by the laboratory that performed your test to evaluate whether your results are “within normal limits.”

For more information, please read the article Reference Ranges and What They Mean.

What is being tested?

Thyroxine (T4) is one of two major hormones produced by the thyroid gland, a small butterfly-shaped organ that lies flat across the windpipe at the base of the throat. The other major thyroid hormone is called triiodothyronine (T3) and together they help control the rate at which the body uses energy. Almost all of the T4 (and T3) found in the blood is bound to protein. The rest is free (unbound) and is the biologically active form of the hormone. This test measures the amount of free T4 in the blood.

Most of the hormone produced by the thyroid is T4. This hormone is relatively inactive, but it is converted into the much more active T3 in the liver and other tissues.

T4 levels are regulated by the feedback system that the body uses to maintain stable amounts of thyroid hormones in the blood. T3 and thyroid stimulating hormone (TSH), along with its regulatory hormone thyrotropin releasing hormone (TRH), which comes from the hypothalamus, are also part of the feedback system.

  • When thyroid hormone levels decrease in the blood, the pituitary gland produces TSH in response to TRH stimulation. TSH in turn stimulates the thyroid by binding to the TSH receptor to produce and release T4 and T3.
  • When thyroid hormone levels increase in the blood, the pituitary gland produces less TSH, and the thyroid produces less T4 and T3.

When all three organs (hypothalamus, pituitary and thyroid) are functioning normally, thyroid production is regulated to maintain relatively stable levels of thyroid hormones in the blood.

If the thyroid gland does not produce sufficient T4 and T3 (underactive thyroid), due to thyroid dysfunction or to insufficient TSH, then the affected person experiences symptoms of hypothyroidism such as weight gain, dry skin, cold intolerance, irregular menstruation, and fatigue. Hashimoto thyroiditis is the most common cause of hypothyroidism in the U.S. (See the article on Hashimoto thyroiditis for more details.)

If the thyroid gland produces too much T4 and T3, the affected person may experience symptoms associated with overactive thyroid (hyperthyroidism), such as rapid heart rate, anxiety, weight loss, difficulty sleeping, tremors in the hands, and puffiness around dry, irritated eyes and in some cases, bulging eyes. Graves disease is the most common cause of hyperthyroidism. (See the article on Graves disease for more details.)

Both hyperthyroidism and hypothyroidism can also be caused by thyroiditis, thyroid cancer, and excessive or deficient production of TSH. The effect of these conditions on thyroid hormone production can be detected and monitored by measuring the free T4.


Common Questions

How is the test used?

Free thyroxine (free T4) tests are used to help evaluate thyroid function and diagnose thyroid diseases, including hyperthyroidism and hypothyroidism, usually after discovering that the thyroid stimulating hormone (TSH) level is abnormal.

Most of the T4 and T3 circulate in the blood bound to protein, while a small percentage is free (unbound). Blood tests can measure total T4, free T4, total T3, or free T3. The total T4 test has been used for many years, but it can be affected by the amount of protein available in the blood to bind to the hormone. Free T4 is not affected by protein levels and is the active form of thyroxine. The free T4 test is thought by many to be a more accurate reflection of thyroid hormone function and, in most cases, its use has replaced that of the total T4 test.

A free T4 test may be used along with or following a TSH test and sometimes with a total or free T3 test to:

  • Help detect too much or too little thyroid hormone (hyperthyroidism and hypothyroidism) and diagnose the cause
  • Distinguish between different thyroid disorders
  • Monitor the effectiveness of treatment in a person with known thyroid disorder
  • Monitor individuals with thyroid cancer, in which the tumors respond to TSH. TSH and T4 levels will be regularly checked to make sure that enough thyroid hormone is being given to keep TSH low without making T4 too high.
  • Help evaluate the function of the pituitary gland (occasionally)

At present, screening the general population for thyroid disorders is not recommended. However, expert opinions vary on this. In 2015, the U.S. Preventive Services Task Force found insufficient evidence to recommend for or against routine screening for thyroid disease in asymptomatic adults.

On the other hand, guidelines released in 2012 by the American Thyroid Association and the American Association of Clinical Endocrinologists summarize recommendations endorsed by several societies. They say that screening for hypothyroidism should be considered in people over the age of 60. Because the signs and symptoms of both hypothyroidism and hyperthyroidism are so similar to those seen in many common disorders, healthcare practitioners often need to rule out thyroid disease even though the patient has another problem.

If a healthcare practitioner suspects that someone may have an autoimmune-related thyroid condition, then thyroid antibodies may be ordered along with a free T4 test.

When is it ordered?

A free T4 test may be ordered when a person has symptoms of hyperthyroidism or hypothyroidism and/or when a person has an enlarged thyroid gland (goiter) or thyroid nodule, particularly if the TSH test is out of range.

Signs and symptoms of hyperthyroidism may include:

  • Increased heart rate
  • Anxiety
  • Weight loss
  • Difficulty sleeping
  • Tremors in the hands
  • Weakness
  • Diarrhea (sometimes)
  • Light sensitivity, visual disturbances
  • There may be puffiness around the eyes with dryness, irritation, and, in some cases, bulging of the eyes.
  • Menstrual irregularity in women

Signs and symptoms of hypothyroidism may include:

  • Weight gain
  • Dry skin
  • Constipation
  • Cold intolerance
  • Puffy skin
  • Hair loss
  • Fatigue
  • Menstrual irregularity in women

Free T4 testing may be ordered along with other thyroid tests on a regular basis when a person is undergoing treatment for a thyroid disorder.

In pregnant women with known thyroid disorders, the healthcare practitioner is likely to order thyroid testing early and late in the pregnancy and for a time period following delivery to monitor the mother and baby.

What does the test result mean?

Free T4 results are typically evaluated with other thyroid testing results (i.e., TSH and sometimes total or free T3). In general, high free T4 results may indicate an overactive thyroid gland (hyperthyroidism), and low free T4 results may indicate an underactive thyroid gland (hypothyroidism). The test results alone are not diagnostic but will prompt the healthcare practitioner to perform additional testing to investigate the cause of the excess or deficiency.

Both decreased and increased free T4 results are associated with a variety of temporary and chronic thyroid conditions. Low free T4 results in conjunction with a low TSH level or high free T4 results along with a high TSH may indicate a pituitary gland condition.

When used for monitoring treatment for thyroid or pituitary disorders, results of thyroid tests will inform the healthcare practitioner whether treatment is effective and/or whether an adjustment to dose is necessary. For example, in people with hyperthyroidism, free T4, free T3, and TSH are regularly checked while they are on anti-thyroid drugs to assure that the drugs are working and to decrease doses if thyroid hormone levels get too low. In people with hypothyroidism, TSH and free T4 are regularly checked to assure that the right dose of thyroid hormone is being given to make TSH normal.

The following table summarizes some examples of typical test results and their potential meaning.

Note: Laboratory results must always be correlated with the clinical findings of the patient.
TSH Free T4 Total or Free T3 Most likely diagnosis
Normal Normal Normal Normal thyroid function (e.g., “euthyroid”)
Normal or decreased Normal or decreased Decreased Normal adjustment in thyroid function due to illness (nonthyroidal illness or sick euthyroid syndrome)
Increased Normal Normal Subclinical hypothyroidism1; in a person with hypothyroidism on treatment, not enough thyroid hormone is being given
Increased Decreased Normal of decreased Hypothyroidism resulting from a problem with the thyroid gland itself (primary hypothyroidism)
Normal or increased Increased Increased Hyperthyroidism resulting from a problem with the pituitary gland signals (central hyperthyroidism) or from a problem with the thyroid hormone receptor (thyroid hormone resistance)
Decreased Normal Normal Subclinical hyperthyroidism2; in a person with hypothyroidism, too much thyroid hormone is being given
Decreased Normal Increased Hyperthyroidism resulting from the thyroid gland making too much active thyroid hormone T3 (uncommon, also known as T3 toxicosis)
Decreased Increased Increased Hyperthyroidism resulting from the gland making too much thyroid hormones (primary hyperthyroidism)
Decreased Decreased Decreased Hypothyroidism resulting from a problem with the hypothalamus or pituitary signals that govern the thyroid gland (central hypothyroidism)

1In affected adults, the diagnosis of subclinical hypothyroidism is applied when the TSH level is elevated and the free T4 level is normal on repeat testing over a number of weeks or months. Adults with subclinical hypothyroidism may have few or no overt symptoms of hypothyroidism. However, subclinical hypothyroidism places affected adults at somewhat increased risk for an elevated LDL cholesterol level, increased risk for cardiovascular disease, and reduced mental acuity.

2In affected adults, the diagnosis of subclinical hyperthyroidism is applied when the TSH level is decreased and the free T4 level and T3 levels are normal on repeat testing over a number of weeks or months. Adults with subclinical hyperthyroidism may have few or no overt symptoms of hyperthyroidism. However, subclinical hyperthyroidism places affected persons at somewhat increased risk for atrial fibrillation and osteoporosis.

Is there anything else I should know?

It is important to note that thyroid tests are a “snapshot” of what is occurring within a dynamic system. An individual person’s results may vary and may be affected by:

  • Increases, decreases, and changes (inherited or acquired) in the proteins that bind T4 and T3 (This is important for tests that measure total T3, but it is not likely important for free T4 and free T3 tests, which measure the thyroid hormones that are not bound to protein.)
  • Pregnancy
  • Liver disease
  • Systemic illness
  • Rarely, resistance to thyroid hormones

What medications can affect thyroid testing?

Many multivitamins, supplements (especially hair and nail), and over-the-counter and prescription medications may affect thyroid test results and their use should be discussed with the healthcare practitioner prior to testing. For example, biotin (vitamin B7) can interfere with some lab tests, so your healthcare practitioner may advise you to refrain from taking biotin or supplements that contain biotin for a few days before having blood drawn for thyroid hormone testing. If you have a procedure done in which fluorescein dyes are injected into your blood (e.g., angiography), you may need to wait a few days before having thyroid testing done.

Do healthcare practitioners test thyroid hormones during pregnancy?

Pregnancy causes normal changes in the function of many endocrine glands, including the thyroid gland. Healthcare practitioners do not generally test asymptomatic women, but those with symptoms and/or a known thyroid disorder will usually be tested at intervals to detect and monitor hyperthyroidism or hypothyroidism during pregnancy and after delivery of the baby.

Some experts have advocated screening pregnant women during the first trimester (or preconception) even if they do not have a history of thyroid disease. However, most guidelines do not support this course of action.

For more information, see the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) webpage: Thyroid Disease and Pregnancy.

My newborn had a free T4 test. Why?

Newborns commonly have T4 levels and TSH levels tested to check for congenital hypothyroidism. Screening for this condition is routinely performed in the United States on newborns soon after birth as part of each state’s newborn screening program. Congenital hypothyroidism occurs when a baby is born with an underactive thyroid gland or a thyroid gland that is not located where it should be or is missing completely. For more details, see the information at the Hormone Health Network’s webpage on Congenital Hypothyroidism.

What is FTI?

FTI stands for the Free Thyroxine Index and it is an estimation of the free T4 concentration. It is sometimes referred to as T7. It is a calculated value determined from the total T4 test and some estimation of the level of thyroid hormone binding proteins. The original test for estimating the level of binding proteins was called the T3-uptake test and later versions were called T-uptake methods. These tests are rarely used now that there are methods available to measure free T4 and T3 directly.

Are there things that I can do to raise or lower my free T4 level?

In general, free T4 does not respond to lifestyle changes. What is important is that the thyroid gland is producing adequate amounts of free T4 and the body’s feedback mechanism is responding appropriately. For those who do not produce enough free T4, thyroid hormone replacement medication can be given.

Health Professionals – LOINC

Logo for LOINC from RegenstriefLOINC Observation Identifiers Names and Codes (LOINC®) is the international standard for identifying health measurements, observations, and documents. It provides a common language to unambiguously identify things you can measure or observe that enables the exchange and aggregation of clinical results for care delivery, outcomes management, and research. Learn More.

Listed in the table below are the LOINC with links to the LOINC detail pages. Please note when you click on the hyperlinked code, you are leaving Lab Tests Online and accessing Loinc.org.

LOINC LOINC Display Name
53349-7 Free T4 (DBS) [Mass/Vol]
3024-7 Free T4 [Mass/Vol]
6892-4 Free T4 Dialysis [Mass/Vol]
83122-2 Free T4 IA [Mass/Vol]
53350-5 Free T4 (DBS) [Moles/Vol]
14920-3 Free T4 [Moles/Vol]
70217-5 Free T4 Dialysis [Moles/Vol]
83121-4 Free T4 IA [Moles/Vol]

View Sources

Sources Used in Current Review

(2017 May. Updated). Thyroid Tests. National Institute of Diabetes and Digestive and Kidney Diseases. Available online at https://www.niddk.nih.gov/health-information/diagnostic-tests/thyroid. Accessed on 6/02/18.

Wisse, B. (2016 February 3, Updated). MedlinePlus Medical Encyclopedia. Available online at https://medlineplus.gov/ency/article/003517.htm. Accessed on 6/02/18.

Sofronescu, A. (2015 November 2, Updated). Thyroid Screen Interpretation. Medscape Protocols. Available online at https://emedicine.medscape.com/article/2172202-overview. Accessed on 6/02/18.

Erik, A. et. al. (2017 March 1). 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum. THYROID Volume 27, Number 3, 2017. Available online at https://www.liebertpub.com/doi/full/10.1089/thy.2016.0457. Accessed on 6/02/18.

Straseski, J. (2018 June, Updated). Thyroid Disease. ARUP Consult. Available online at https://arupconsult.com/content/thyroid-disease. Accessed on 6/02/18.

(© 2018). Thyroid Function Tests. American Thyroid Association. Available online at https://www.thyroid.org/thyroid-function-tests/. Accessed on 6/02/18.

Schneider, C. (2018 April 30). Initial evaluation of thyroid dysfunction – Are simultaneous TSH and fT4 tests necessary? PLoS One. 2018; 13(4): e0196631. Available online at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5927436/. Accessed on 6/02/18.

Ross, D. et. al. (2016 October). 2016 American Thyroid Association Guidelines for Diagnosis and Management of Hyperthyroidism and Other Causes of Thyrotoxicosis. Thyroid Volume 26, Issue 10. Available online at https://www.liebertpub.com/doi/full/10.1089/thy.2016.0229. Accessed on 6/02/18.

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