When you have symptoms of an underactive (hypothyroidism) or overactive (hyperthyroidism) thyroid gland
A blood sample taken from a vein in the arm
No test preparation is necessary
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.
How is it used?
When is it requested?
What does the test result mean?
Is there anything else I should know?
Many medicines including oestrogen, 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 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.
How is hyperthyroidism treated?
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.
How is hypothyroidism treated?
How does pregnancy affect thyroid hormone levels?