FT4
Note: this site is for informational purposes only. To view test results or book a test, use the NHS app in England or contact your GP.
The FT4 (free thyroxine) test measures the level of free thyroxine (FT4) in a blood sample, one of the main hormones produced by the thyroid gland. It is used to assess thyroid function and help diagnose or monitor thyroid disorders, such as hyperthyroidism and hypothyroidism
Why get tested?
To diagnose hypothyroidism or hyperthyroidism in adults and to monitor response to treatment.
When to get tested?
When you have symptoms of an underactive (hypothyroidism) or overactive (hyperthyroidism) thyroid gland.
Sample required?
A blood sample taken from a vein in the arm.
Test preparation needed?
No test preparation is necessary.
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 (T3)). Within the blood, most thyroid hormones are attached to a specific 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’ thyroxine 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 specific protein; less than 1% is unattached. Blood tests can measure either the total (both bound and unattached, TT4) or free (unattached, FT4) T4 hormone in your blood. Most laboratories measure FT4, as it is the free hormone that is thought to be responsible for all the effects of thyroid hormone within the body.
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 necessary.
Common questions
FT4 can tell whether the thyroid is functioning adequately. It aids in the diagnosis of hypothyroidism or hyperthyroidism. The test may also be used to help evaluate a patient with an enlarged thyroid gland, called a goitre. It may also aid in the diagnosis of female infertility problems.
FT4 test is usually used in response to an abnormal TSH test result.
High free T4 results along with appropriate clinical symptoms may indicate an overactive thyroid gland (hyperthyroidism).
Low free T4 results along with appropriate clinical symptoms may indicate an underactive thyroid gland (hypothyroidism).
Many medicines including , certain types of contraceptive birth control pills, those drugs used to help control epilepsy and large doses of aspirin could interfere with total T4 test results, so tell your doctor about any drugs/prescribed medications/supplements you are taking. In general, free T4 levels are minimally affected by these medications, another reason why FT4 is routinely used to assess thyroid function.
Hyperthyroidism can be controlled through treatment. This will normally involve either tablets which stop the thyroid gland producing thyroid hormones, or radioiodine treatment which destroys thyroid tissue, or surgery to remove part or all of the thyroid gland.
Hypothyroidism is easily treated and controlled for most people with thyroxine (T4) replacement in the form of a tablet.
During pregnancy total T4 levels can increase. Normally, free T4 levels will not change significantly as a result of pregnancy. Even if total T4 levels rise during pregnancy, it does not mean thyroid disease exists and these changes usually resolve after delivery of the baby.