Thyroxine (T4) Test Understand the Test & Your Results (2024)

Accordion Title

  • What is a thyroxine (T4) test?

    A thyroxine test helps diagnose disorders of the thyroid. The thyroid is a small, butterfly-shaped gland located near the throat. Your thyroid makes hormones that regulate the way your body uses energy. It also plays an important role in regulating your weight, body temperature, muscle strength, and even your mood. Thyroxine, also known as T4, is a type of thyroid hormone. This test measures the level of T4 in your blood. Too much or too little T4 can indicate thyroid disease .

    The T4 hormone comes in two forms:

    • Free T4, which enters the body tissues where it's needed
    • Bound T4, which attaches to proteins, preventing it from entering body tissues

    A test that measures both free and bound T4 is called a total T4 test. Other tests measure just free T4. A free T4 test is considered more accurate than a total T4 test for checking thyroid function.

    Other names: free thyroxine, free T4, total T4 concentration, thyroxine screen, free T4 concentration

  • What is it used for?

    A T4 test is used to evaluate thyroid function and diagnose thyroid disease.

  • Why do I need a thyroxine test?

    Thyroid disease is much more common in women and most often occurs under the age of 40. It also tend to run in families. You may need a thyroxine test if a family member has ever had thyroid disease or if you have symptoms of having too much thyroid hormone in your blood, a condition called hyperthyroidism , or symptoms of having too little thyroid hormone, a condition called hypothyroidism .

    Symptoms of hyperthyroidism, also known as overactive thyroid, include:

    • Anxiety
    • Weight loss
    • Tremors in the hands
    • Increased heart rate
    • Puffiness
    • Bulging of the eyes
    • Trouble sleeping

    Symptoms of hypothyroidism, also known as underactive thyroid, include:

    • Weight gain
    • Fatigue
    • Hair loss
    • Low tolerance for cold temperatures
    • Irregular menstrual periods
    • Constipation
  • What happens during a thyroxine test?

    A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.

  • Will I need to do anything to prepare for the test?

    You don't need any special preparations for a thyroxine blood test. If your health care provider has ordered more tests on your blood sample, you may need to fast (not eat or drink) for several hours before the test. Your health care provider will let you know if there are any special instructions to follow.

  • Are there any risks to the test?

    There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.

  • What do the results mean?

    Your results may come in the form of total T4, free T4, or a free T4 index.

    • The free T4 index includes a formula that compares free and bound T4.
    • High levels of any of these tests (total T4, free T4, or free T4 index) may indicate an overactive thyroid, also known as hyperthyroidism.
    • Low levels of any of these tests (total T4, free T4, or free T4 index) may indicate an underactive thyroid, also known as hypothyroidism.

    If your T4 test results are not normal, your health care provider will likely order more thyroid tests to help make a diagnosis. These may include:

    • T3 thyroid hormone tests. T3 is another hormone made by the thyroid.
    • A TSH (thyroid stimulating hormone) test. TSH is a hormone made by the pituitary gland. It stimulates the thyroid to produce T4 and T3 hormones.
    • Tests to diagnose Graves' disease, an autoimmune disease that causes hyperthyroidism
    • Tests to diagnose Hashimoto's thyroiditis, an autoimmune disease that causes hypothyroidism

    Learn more about laboratory tests, reference ranges, and understanding results .

  • Is there anything else I need to know about a thyroxine test?

    Thyroid changes can happen during pregnancy . Although it is not common, some women can develop thyroid disease during pregnancy. Hyperthyroidism happens in about 0.1% to 0.4% of pregnancies, while hypothyroidism happens in approximately 2.5% of pregnancies.

    Hyperthyroidism, and less often, hypothyroidism, may remain after pregnancy. If you develop a thyroid condition during pregnancy, your health care provider will monitor your condition after your baby is born. Also, if you have a history of thyroid disease, be sure to talk with your health care provider if you are pregnant or are thinking of becoming pregnant.

  • References
    1. American Thyroid Association [Internet]. Falls Church (VA): American Thyroid Association; c2017. Thyroid Function Tests [cited 2017 May 22]; [about 2 screens]. Available from: https://www.thyroid.org/thyroid-function-tests
    2. Hinkle J, Cheever K. Brunner & Suddarth's Handbook of Laboratory and Diagnostic Tests. 2nd Ed, Kindle. Philadelphia: Wolters Kluwer Health, Lippincott Williams & Wilkins; c2014. Thryoxine, Serum 485 p.
    3. Lab Tests Online [Internet]. Washington D.C.: American Association for Clinical Chemistry; c2001–2017. Free T4: The Test [updated 2014 Oct 16; cited 2017 May 22]; [about 4 screens]. Available from: https://labtestsonline.org/understanding/analytes/t4/tab/test
    4. Lab Tests Online [Internet]. Washington D.C.: American Association for Clinical Chemistry; c2001–2017. Free T4: The Test Sample [updated 2014 Oct 16; cited 2017 May 22]; [about 3 screens]. Available from: https://labtestsonline.org/understanding/analytes/t4/tab/sample
    5. Lab Tests Online [Internet]. Washington D.C.: American Association for Clinical Chemistry; c2001–2017. TSH: The Test Sample [updated 2014 Oct 15; cited 2017 May 22]; [about 3 screens]. Available from: https://labtestsonline.org/understanding/analytes/tsh/tab/sample
    6. Merck Manual Consumer Version [Internet]. Kenilworth (NJ): Merck & Co. Inc.; c2017. Overview of the Thyroid Gland [cited 2017 May 22]; [about 2 screens]. Available from: https://www.merckmanuals.com/home/hormonal-and-metabolic-disorders/thyroid-gland-disorders/overview-of-the-thyroid-gland
    7. National Institute of Diabetes and Digestive and Kidney Diseases [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; Graves' Disease; 2012 Aug [cited 2017 May 22]; [about 3 screens]. Available from: https://www.niddk.nih.gov/health-information/endocrine-diseases/graves-disease
    8. National Institute of Diabetes and Digestive and Kidney Diseases [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; Hashimoto's Disease; 2014 May [cited 2017 May 22]; [about 3 screens]. Available from: https://www.niddk.nih.gov/health-information/endocrine-diseases/hashimotos-disease
    9. National Institute of Diabetes and Digestive and Kidney Diseases [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; Thyroid Tests; 2014 May [cited 2017 May 22]; [about 3 screens]. Available from: https://www.niddk.nih.gov/health-information/diagnostic-tests/thyroid
    10. National Heart, Lung, and Blood Institute [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; What Are the Risks of Blood Tests? [updated 2012 Jan 6; cited 2017 May 22]; [about 5 screens]. Available from: https://www.nhlbi.nih.gov/health-topics/blood-tests#Risk-Factors
    11. National Heart, Lung, and Blood Institute [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; What To Expect with Blood Tests [updated 2012 Jan 6; cited 2017 May 22]; [about 4 screens]. Available from: https://www.nhlbi.nih.gov/health-topics/blood-tests
    12. Soldin OP. Thyroid Function Testing in Pregnancy and Thyroid Disease: Trimester-specific Reference Intervals. Ther Drug Monit. [Internet]. 2006 Feb [cited 2019 Jun 3]; 28(1):8-11. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3625634
    13. University of Rochester Medical Center [Internet]. Rochester (NY): University of Rochester Medical Center; c2017. Health Encyclopedia: Free and Bound T4 [cited 2017 May 22]; [about 2 screens]. Available from: https://www.urmc.rochester.edu/encyclopedia/content.aspx?contenttypeid=167&contentid;=t4_free_and_bound_blood
    14. University of Rochester Medical Center [Internet]. Rochester (NY): University of Rochester Medical Center; c2017. Health Encyclopedia: Free T4 [cited 2017 May 22]; [about 2 screens]. Available from: https://www.urmc.rochester.edu/encyclopedia/content.aspx?contenttypeid=167&contentid=free_t4_thyroxine
Thyroxine (T4) Test Understand the Test & Your Results (2024)

FAQs

Thyroxine (T4) Test Understand the Test & Your Results? ›

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.

How do you read T4 lab results? ›

Understanding T4 Blood Test Results
  1. Children up to 5 years old: 0.8-2.8 nanograms per deciliter (ng/dL)
  2. Children 6-15 years old: 0.8-2.1 ng/dL.
  3. Teens 16-17 years old assigned male at birth: 0.8-2.8 ng/dL.
  4. Teens 16-17 years old assigned female at birth: 0.8-1.5 ng/dL.
  5. Adults over 18 years old: 0.9-1.7 ng/dL.
Mar 16, 2024

How to understand thyroid test results? ›

Thyroid-stimulating hormone (TSH): High TSH levels are a sign of an underactive thyroid (hypothyroidism). The pituitary gland produces more TSH in order to stimulate the thyroid gland to produce thyroid hormones. Very low TSH levels in the blood may be a sign of an overactive thyroid (hyperthyroidism).

What is a good thyroxine T4 level? ›

Normal Values:

Normal values vary among different laboratories. A typical normal range is: 4.5 to 11.2 mcg/dL (micrograms per deciliter).

Is it better to have high or low free T4? ›

The normal range for free T 4 in adults is 0.8 to 1.8 nanograms per deciliter (ng/dL). A level of free T 4 that is higher than normal could mean you have an overactive thyroid. Conditions linked to hyperthyroidism include Graves disease, an autoimmune disorder.

What is a normal T4 score? ›

A normal Total T4 level in adults ranges from 5.0 to 12.0μg/dL. A normal Total T3 level in adults ranges from 80-220 ng/dL. Free T3 assays are often unreliable and not routinely used to assess thyroid function.

What number should my thyroid blood test results mean? ›

TSH levels typically fall between 0.4 and 4.0 mU/L, according to the American Thyroid Association. Ranges may vary with labs, with the upper limit generally being between 4 and 5. If your TSH level is higher than this, chances are you have an underactive thyroid.

What is a bad number for thyroid? ›

What levels of TSH are concerning? TSH levels below 0.4mU/L indicate hyperthyroidism, while levels of about 4.0mU/L and above indicate hypothyroidism. The further the result from these levels, the more severe the result is.

What is a good thyroid reading? ›

It's often the most sensitive indicator that a thyroid problem is present. The normal range of TSH levels in adults is between 0.4 to 4.0 mIU/L (milli-international units per liter). Some research suggests that this range should actually be more like 0.45 to 2.5 mIU/L.

How do you read thyroid function? ›

The finding of an elevated TSH and low FT4 or FTI indicates primary hypothyroidism due to disease in the thyroid gland. A low TSH and low FT4 or FTI indicates hypothyroidism due to a problem involving the pituitary gland. A low TSH with an elevated FT4 or FTI is found in individuals who have hyperthyroidism.

What is critical level of T4? ›

Critical Values (Total T4):

Newborn: < 7 mcg/dL. Adult: < 2 mcg/dL where myxedema coma is possible and if > 20 mcg/dL then thyroid storm possible.

What do free T4 results mean? ›

Free T4 is a hormone produced by the thyroid gland. T4 helps regulate metabolism and body temperature. Elevated Free T4 results may indicate hyperthyroidism. Low Free T4 results may indicate hypothyroidism.

What is the normal T4 level for age? ›

Normal levels of free T4 vary based on your age. In general, normal ranges of free T4 for healthy people include: Children up to 5 years old: 0.8 – 2.8 nanograms per deciliter (ng/dL). Children 6 to 15 years old: 0.8 – 2.1 ng/dL.

What test confirms Hashimoto's disease? ›

To determine if Hashimoto's disease is the cause of hypothyroidism, your health care provider will order an antibody test. The intended purpose of an antibody is to flag disease-causing foreign agents that need to be destroyed by other actors in the immune system.

What happens if your T4 is too high? ›

In general, T4 results that are higher than normal may be a sign of: Hyperthyroidism, which may be caused by Graves disease or another medical condition that causes your thyroid to make too much T4. Thyroiditis (thyroid inflammation) Toxic goiter (an enlarged thyroid with areas that make extra thyroid hormone)

What is a normal thyroid level for a woman? ›

TSH normal range for females typically falls within the range of 0.5 to 5.0 mIU/L. However, TSH levels can fluctuate due to factors such as hormonal changes during the menstrual cycle and pregnancy. Pregnancy also causes hormonal fluctuations.

What is the normal value of T3, T4, and TSH? ›

Normal biological value: A TSH reading is normal if the result is in the range of 0.4 to 5 mIU/L (milli-international units per liter). Normal T3 T4 index (for adults) when T3 hormone reaches 1.3 - 3.1 nmol/l or 0.8-2.0 ng/ml.

What is the normal range for T4 by age? ›

Children 6 to 15 years old: 0.8 – 2.1 ng/dL. Adolescents assigned male at birth 16 to 17 years old: 0.8 – 2.8 ng/dL. Adolescents assigned female at birth 16 to 17 years old: 0.8 – 1.5 ng/dL. Adults over 18 years old: 0.9 – 1.7 ng/dL.

What does T4 total mean in a blood test? ›

What Is T4? T4, or thyroxine, is a hormone that is made by the thyroid gland and helps control metabolism and growth. T4 may be referred to as: total T4, which is the entire amount of thyroxine in the blood, including bound T4, which attaches to proteins. free T4, which is only free T4, which isn't attached to proteins.

References

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