Formal Name
Free Thyroxine
This article was last reviewed on
This article waslast modified on 29 January 2018.
At a Glance
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

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 may be able to access your results online. Your GP practice will be able to provide specific details.

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, sex, 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?

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.

Accordion Title
Common Questions
  • How is it used?

    FT4 can tell whether the thyroid is performing properly. 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.

  • When is it requested?

    FT4 test is usually used in response to an abnormal TSH test result.

  • What does the test result mean?

    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).

  • 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?

    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 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.