At a Glance
Why Get Tested?
To help diagnose hyperthyroidism
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 that you are taking
The Test Sample
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 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.
How is the sample collected for testing?
A blood sample is obtained from a needle placed in a vein in your arm.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
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
The Test
How is it used?
A FT3 blood test determines whether the thyroid is performing properly, and is used mainly to help diagnose hyperthyroidism, since T3 can become abnormal earlier than T4 and return to normal later than T4.
FT3 is not usually helpful if your doctor thinks you have hypothyroidism.
When is it requested?
What does the test result mean?
A high total or free T3 result may indicate an overactive thyroid gland (hyperthyroidism).
A low total or free T3 results may indicate an underactive thyroid gland (hypothyroidism).
Is there anything else I should know?
Many medications—including oestrogen, 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 T3 or free T3 concentration in their blood. For this reason, doctors do not usually use FT3 as a routine thyroid test for patients in hospitals.
Common Questions
Ask a Laboratory Scientist
Article Sources
NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
