At a glance
Also known as
Free T4; T4
Why get tested?
To diagnose or in adults
When to get tested?
FT4 may be is measured in response to an abnormal TSH test result
A blood sample drawn from a vein in the arm.
Test preparation needed?
None but tell your doctor what medications you are taking as some may affect the test results.
What is being tested?
This test measures the amount of free thyroxine, or FT4, in your blood. T4 is one of two major produced by the thyroid gland (the other is called triiodothyronine, or T3).
T4 makes up nearly all of what we call thyroid hormone, while T3 makes up less than 10%. Thyroid hormones help regulate the body’s (that is, how the body functions).
Most T4 in blood is attached to a ; less than 1% is unattached. The blood test measures the amount of free (unattached) T4 hormone in your blood since this is the biologically relevant fraction.
Scientists believe that free hormone is responsible for all the effects of thyroid hormone.
There is a system between the pituitary gland which produces TSH and the thyroid. Normally TSH produced in the pituitary drives thyroid T3 and T4 production. If thyroid hormone production falls, TSH rises and if T3 and T4 become too high, TSH levels fall.
How is the sample collected for testing?
A blood sample is obtained from a needle placed in a vein in your arm.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.