Formal Name
Free triiodothyronine
This article was last reviewed on
This article waslast modified on
24 April 2018.
At a Glance
Why Get Tested?

To help diagnose hyperthyroidism and to monitor it's treatment

When To Get Tested?

If you get an abnormal thyroid stimulating hormone (TSH) or free thyroxine (FT4) result

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

On average it takes 7 working days for the blood test results to come back from the hospital, depending on the exact tests requested. Some specialist test results may take longer, if samples have to be sent to a reference (specialist) laboratory. The X-ray & scan results may take longer. If you are registered to use the online services of your local practice, you will be able to access your results online.

If the doctor wants to see you about the result(s), you will be offered an appointment. If you are concerned about your test results, you will need to arrange an appointment with your doctor so that all relevant information including age, ethnicity, health history, signs and symptoms, laboratory and other procedures (radiology, endoscopy, etc.), can be considered.

Lab Tests Online-UK is an educational website designed to provide patients and carers with information on laboratory tests used in medical care. We are not a laboratory and are unable to comment on an individual's health and treatment.

Reference ranges are dependent on many factors, including patient age, gender, sample population, and test method, and numeric test results can have different meanings in different laboratories.

For these reasons, you will not find reference ranges for the majority of tests described on this web site. The lab report containing your test results should include the relevant reference range for your test(s). Please consult your doctor or the laboratory that performed the test(s) to obtain the reference range if you do not have the lab report.

For more information on reference ranges, please read Reference Ranges and What They Mean.

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

Accordion Title
Common Questions
  • How is it used?

    A FT3 blood test determines whether the thyroid is performing properly, 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.

  • When is it requested?

    A free T3 blood test may be performed if you get an abnormal TSH or FT4 test result.

  • What does the test result mean?

    A high free T3 result may indicate an overactive thyroid gland (hyperthyroidism).

    A low 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 free T3 concentration in their blood. For this reason, doctors do not usually use FT3 as a routine thyroid test for patients in hospitals.

  • How is hyperthyroidism treated?

    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.

  • How is hypothyroidism treated?

    Hypothyroidism is easily treated and controlled for most people with thyroxine (T4) replacement in the form of a tablet.

  • How does pregnancy affect thyroid hormone levels?

    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.