At a glance
Also known as
Free T4; T4; free thyroxine
Why get tested?
To help diagnose and monitor treatment of thyroid disorders
When to get tested?
Thyroid stimulating hormone is the preferred initial test in the assessment of thyroid function. FT4 may be 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 per cent. Thyroid hormones help regulate the body’s (that is, how the body functions).
Most T4 in blood is attached to a ; less than 1 per cent is unattached. The blood test measures the amount of free (unattached) T4 hormone in your blood since this is the biologically relevant fraction.
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 but tell your doctor what medications you are taking as some may affect the test results.