Formal Name
Free Thyroxine
This article was last reviewed on
This article waslast modified on 3 January 2025.
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...

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