At a Glance
Why Get Tested?
When to Get Tested?
Usually ordered in response to an abnormal TSH test result or when a person has symptoms of hyperthyroidism or hypothyroidism; commonly performed as a screening test on newborns
A blood sample drawn from a vein in the arm or from pricking the heel of an infant
Test Preparation Needed?
None needed; however, certain medications can interfere with the T4 test, so tell you doctor about any drugs that you are taking.
The Test Sample
What is being tested?
This test measures the amount of thyroxine, or T4, in the blood. T4 is one of two major hormones produced by the thyroid gland; the other is called triiodothyronine, or T3. The thyroid is a small, butterfly-shaped gland located just below the Adam's apple. This gland plays a vital role in controlling metabolism, the rate energy is used.
The body has a feedback system that turns thyroid hormone production on and off. When the level of T4 in the bloodstream decreases, the hypothalamus releases thyrotropin releasing hormone, which stimulates the pituitary gland to release thyroid-stimulating hormone (TSH), which in turn stimulates the thyroid gland to make and/or release more T4. As blood concentrations of T4 increase, TSH release is inhibited.
T4 makes up about 90% of thyroid hormones. When the body requires thyroid hormone, the thyroid gland releases stored T4 into circulation. In the blood, T4 is either free (not bound) or protein-bound (primarily bound to thyroxine-binding globulin). The concentration of free T4 is only about 0.1% of that of total T4. T4 is converted into T3 in the liver or other tissues. T3, like T4, is also highly protein-bound, but it is the free forms of T3 and T4 that are biologically active. Free T3 is 4 to 5 times more active than free T4 in circulation.
If the thyroid gland does not produce sufficient T4, 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. If the thyroid gland produces too much T4, the rate of the person's body functions will increase and cause symptoms associated with hyperthyroidism such as increased heart rate, anxiety, weight loss, difficulty sleeping, tremors in the hands, and puffiness around dry, irritated eyes.
The most common causes of thyroid dysfunction are autoimmune-related. Graves disease causes hyperthyroidism and Hashimoto thyroiditis causes hypothyroidism. 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 total T4 (includes bound and free portion) or the free T4 (unbound form).
How is the sample collected for testing?
A blood sample is obtained from a needle placed in a vein in the arm or from pricking the heel of an infant.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed. Certain medications can interfere with the T4 test, so tell the doctor about any drugs that you are taking.
Ask a Laboratory Scientist
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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
Sources Used in Current Review
Eckman, A. (Updated 2010 April 20). T4 Test. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003517.htm. Accessed February 2011.
Pagana, K. D. & Pagana, T. J. (© 2011). Mosby's Diagnostic and Laboratory Test Reference 10th Edition: Mosby, Inc., Saint Louis, MO. Pp 958-960.
Lee, S. and Ananthakrishnan, S. (Updated 2010 April 26). Hyperthyroidism. eMedicine [On-line information]. Available online at http://emedicine.medscape.com/article/121865-overview through http://emedicine.medscape.com. Accessed February 2011.
(© 2008). Thyroid Function Tests. American Thyroid Association [On-line information]. Available online at http://www.thyroid.org/patients/patient_brochures/function_tests.html through http://www.thyroid.org. Accessed February 2011.
Irizarry, L. et. al. (Updated 2010 April 23). Toxicity, Thyroid Hormone. eMedicine [On-line information]. Available online at http://emedicine.medscape.com/article/819692-overview through http://emedicine.medscape.com. Accessed February 2011.
Sources Used in Previous Reviews
Thomas, Clayton L., Editor (1997). Taber's Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].
Pagana, Kathleen D. & Pagana, Timothy J. (2001). Mosby's Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO.
U.S. Preventive Services Task Force (2004 May 15). Screening for Thyroid Disease: Recommendation Statement. American Family Physician [On-line journal]. Available online at http://www.aafp.org/afp/20040515/us.html through http://www.aafp.org.
MedlinePlus Medical Encyclopedia. T4 (Updated 10/24/07). [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003517.htm.
(© 2005). Thyroxine, Free (Free T4). ARUP's Guide to Clinical Laboratory Testing [On-line information]. Available online at http://www.aruplab.com/guides/clt/tests/clt_245b.jsp#1149312 through http://www.aruplab.com.
(© 2005). Thyroxine. ARUP's Guide to Clinical Laboratory Testing [On-line information]. Available online at http://www.aruplab.com/guides/clt/tests/clt_243b.jsp#1149270 through http://www.aruplab.com.
Wu, A. (2006). Tietz Clinical Guide to Laboratory Tests, Fourth Edition. Saunders Elsevier, St. Louis, Missouri. Pp. 1050-1053.
Amarillo Medical Specialists. How to interpret your blood test results. Available online at http://www.amarillomed.com/howto.htm through http://www.amarillomed.com.