At a Glance
Why Get Tested?
When to Get Tested?
When you have symptoms of an underactive (hypothyroidism) or overactive (hyperthyroidism) thyroid gland
A blood sample taken from a vein in the arm
Test Preparation Needed?
No test preparation is necessary
The Test Sample
What is being tested?
This test measures free thyroxine, or FT4, in your blood. Thyroxine is one of two major hormones produced by the thyroid gland (the other is called triiodothyronine. Within the blood, most thyroid hormones are attached to a protein, but it is the hormones that are free from these proteins that are able to affect body functions. This is therefore why we measure the 'free' thyroixine in your blood sample. The thyroid gland is found in the neck, in front of the windpipe.
Thyroxine (T4) makes up nearly all of what we call thyroid hormone, while triiodothyronine (T3) makes up less than 10%. Thyroid hormones help regulate the body’s metabolism (that is, how the body functions).
Most T4 in blood is attached to a protein; less than 1% is unattached. The blood test can measure either the total (both bound and unattached, TT4) or free (unattached, FT4) T4 hormone in your blood.
Scientists believe that free hormone is responsible for all the effects of thyroid hormone.
How is the sample collected for testing?
A blood sample is obtained from a needle placed in a vein in your arm.
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 necessary.
Ask a Laboratory Scientist
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.