FT3
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 FT3 (free triiodothyronine) test measures the level of free T3 hormone in a blood sample to assess the amount of active thyroid hormone available in the body. It is used to help evaluate thyroid function and diagnose or monitor thyroid disorders, such as hyperthyroidism or hypothyroidism, often alongside other thyroid tests.
Why get tested?
To help diagnose hyperthyroidism and to monitor it’s treatment.
When to get tested?
Sample required?
A blood sample taken from a vein in the arm.
Test preparation needed?
None needed; however, certain medications can interfere with the FT3 test, so tell your doctor about any drugs or prescribed medications that you are taking, including any over-the-counter supplements.
What is being tested?
The test measures the amount of free triiodothyronine, or FT3, in your blood.
T3 is one of two major hormones produced by the thyroid gland (the other hormone is called thyroxine, or T4). The thyroid gland is found in the neck, in front of the windpipe. T3 makes up less than 10% of what we call thyroid hormone, while T4 makes up the rest. T3, however, is about four times as strong as T4, and is thought to cause most, if not all, the effects of thyroid hormones.
Many of your body’s cells can turn T4 into T3; T4 may be mainly a “reservoir” used to make T3 available.
Thyroid hormones help regulate the body’s metabolism (how the body functions).
About 99.7% of T3 in blood is attached to a specific protein, and the rest is unattached (free). Blood tests can measure either the total (both bound and unattached) or free T3 hormone in the blood. These tests are called TT3 (total T3), and FT3 (free T3) respectively. Most laboratories will only routinely measure and report FT3 results.
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?
None needed; however, certain medications can interfere with the FT3 test, so tell your doctor about any drugs that you are taking.
Common questions
A FT3 blood test determines whether the thyroid is functioning adequately, and is used mainly to help diagnose hyperthyroidism, since FT3 can become abnormally high earlier than FT4 and return to normal later than FT4.
FT3 is not usually helpful if your doctor thinks you have hypothyroidism.
A high free T3 result may indicate an overactive thyroid gland (hyperthyroidism).
A low free T3 results may indicate an underactive thyroid gland (hypothyroidism).
Many medications including certain types of contraceptive birth control pills, and large doses of aspirin can interfere with total T3 test results, so tell your doctor about any drugs you are taking. However, in general, free T3 concentrations are affected much less by these medications.
When you are sick, your body decreases production of T3 from T4. Most people who are sick enough to be in the hospital will have a low free T3 concentration in their blood. For this reason, doctors do not usually use FT3 as a routine thyroid test for patients in hospitals.
Hyperthyroidism can be controlled through treatment. This will normally involve either tablets which stop the thyroid gland producing thyroid hormones, 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 T3 (and total T4) levels can increase. Normally, free T3 levels will not change significantly as a result of pregnancy. Even if total T3 levels rise during pregnancy, it does not mean thyroid disease will develop.