FT3 & FT4 (Free T3 & Free T4)
Understanding FT3 & FT4 (Free T3 & Free T4)
What is FT3 & FT4 (Free T3 & Free T4)?
A FT3 & FT4 (Free T3 & Free T4) test measures the amount of free form of triiodothyronine (T3) and thyroxine (T4) hormones in the blood. This test helps evaluate the functioning of the thyroid gland and detect possible thyroid disorders, including hyperthyroidism and hypothyroidism.
The thyroid gland is a butterfly-shaped organ located in the neck that produces the hormones thyroxine (T4) and triiodothyronine (T3) which are responsible for regulating metabolism and other bodily functions. T3 and T4 circulate in the blood in two forms: bound form (attached to proteins) and free form (does not attach to any protein). The production and release of these hormones are controlled by the thyroid-stimulating hormone (TSH) which is produced by the pituitary gland.
Deranged levels of T3 and T4 hormones can result in either hyperthyroidism (a condition in which the thyroid gland produces too much T3 and T4 hormones) or hypothyroidism (a condition in which the thyroid gland produces less T3 and T4 hormones). Your doctor may advise a FT3 & FT4 (Free T3 & Free T4) test if you show symptoms suggestive of hyperthyroidism, such as increased appetite, weight loss, sweating, anxiety, tremors, etc., or hypothyroidism, such as fatigue, weight gain, dry skin, constipation, etc.
No special preparations are needed for this test. Usually, you do not need to stop taking your thyroid medication before taking this test. However, do let your doctor know about all the medications you take as some of them can affect your thyroid function test results. Biotin (Vitamin B7) is a commonly taken over-the-counter supplement that can lead to some aberrancy in your test results even when the values are actually normal. Therefore, avoid taking biotin 2 days before an FT3 & FT4 test.
Test result ranges are approximate and may differ slightly between different labs depending on the methodology and laboratory guidelines. Talk to your doctor about your specific results. The test results will help them determine your medical condition, make recommendations for lifestyle modifications such as diet and exercise, evaluate whether or not medication will be required to manage your condition, and formulate an overall treatment plan.
What is FT3 & FT4 (Free T3 & Free T4) used for?
A FT3 & FT4 (Free T3 & Free T4) test is done:
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To measure the levels of free T3 and T4 hormones in the body.
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As part of a thyroid profile test along with a TSH test.
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To evaluate symptoms suggestive of an imbalance in thyroid hormone levels, such as fatigue, weight loss, hair loss, and changes to heart rate.
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To diagnose and monitor the treatment of hypothyroidism or hyperthyroidism.
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To detect an underactive thyroid gland in newborn babies.
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To evaluate other disorders, such as thyroid nodules, goiters, and pituitary or hypothalamus gland disorders.
What does FT3 & FT4 (Free T3 & Free T4) measure?
A FT3 & FT4 (Free T3 & Free T4) test measures the free form of triiodothyronine (FT3) and thyroxine (FT4) hormones made by the thyroid gland. These hormones exist in the blood in two forms: bound (attached to proteins) and free (not attached to proteins). Most of the T3 and T4 hormones circulating in the blood are bound to proteins and only a small part circulates freely and constitutes the physiologically active form. The body maintains a delicate balance of bound and free T3 as well as T4 to ensure proper functioning.
Answers to Patient Concerns & Frequently Asked Questions (FAQs) about FT3 & FT4 (Free T3 & Free T4)
Frequently Asked Questions about FT3 & FT4 (Free T3 & Free T4)
Q. Why is a FT3 & FT4 (Free T3 & Free T4) test done?
Q. Can I take a FT3 & FT4 (Free T3 & Free T4) test during pregnancy?
Q. What happens when T3 and T4 levels are high?
Q. What happens when T3 and T4 levels are low?
Q. Is fasting required for a FT3 & FT4 (Free T3 & Free T4) test?
Q. What factors can affect the results of FT3 & FT4 (Free T3 & Free T4) test?
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References
- Thyroid Function Tests [Internet]. Alexandria, VA: American Thyroid Association; [Accessed 10 Jul. 2023]. Available from: https://www.thyroid.org/thyroid-function-tests/
- Free and Bound Triiodothyronine (Blood) [Internet]. NY: University of Rochester Medical Center; [Accessed 10 Jul. 2023]. Available from: https://www.urmc.rochester.edu/encyclopedia/content.aspx?ContentTypeID=167&ContentID=t3_free_and_bound_blood
- Welsh KJ, Soldin SJ. Diagnosis of endocrine disease: How reliable are free thyroid and total T3 hormone assays?. Eur J Endocrinol. 2016 Dec;175(6):R255-R263. [Accessed 10 Jul. 2023]. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5113291/
- Block-Galarza J. Thyroid Function Tests [Internet]. Alexandria, VA: American Thyroid Association. [Accessed 10 Jul. 2023]. Available from: https://www.thyroid.org/patient-thyroid-information/ct-for-patients/december-2018/vol-11-issue-12-p-3-4/#:~:text=Most%20commonly%2C%20biotin%20use%20can,hormone%20dose%20is%20too%20high
- Thyroid Tests [Internet]. NIDDK; May 2017 [Accessed 10 Jul. 2023]. Available from: https://www.niddk.nih.gov/health-information/diagnostic-tests/thyroid
- Adhimoolam M, Arulmozhi R. Effect of antiepileptic drug therapy on thyroid hormones among adult epileptic patients: An analytical cross-sectional study. J Res Pharm Pract. 2016 Jul-Sep;5(3):171-4. [Accessed 10 Jul. 2023]. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4966235/
- Dong BJ. How medications affect thyroid function. West J Med. 2000 Feb;172(2):102-6. [Accessed 10 Jul. 2023]. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1070767/
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