WellSpan Home

Health Library

Free T4

Does this test have other names?

Free thyroxine test

What is this test?

This test measures the levels of free T4, or free thyroxine, in your blood. A free T4 test is used to find out how well your thyroid is working.

T4 is one of two hormones produced by the thyroid, a butterfly-shaped gland in your neck. Some T4 in your blood is bound to proteins, and some T4 circulates freely, or unbound from proteins. Testing for unbound T4 is more accurate than testing for bound T4. The free T4 test measures unbound T4.

The other thyroid hormone is triiodothyronine, or T3. T4 is changed to T3 in order to become active and effective.

These hormones help regulate your body's metabolism. They go into action when prompted by thyroid stimulating hormone (TSH), which is released by the pituitary gland in your brain.

Why do I need this test?

You may need this test to find out whether you have a thyroid-related condition such as hyperthyroidism, which means an overactive thyroid, or hypothyroidism, which means an underactive thyroid.

Each condition has many different symptoms. If you have hyperthyroidism, you may often feel anxious and irritable, have trouble sleeping, and have an irregular or rapid heart rhythm. You may also feel quite tired and notice that you're losing weight even though your appetite has increased.

If you have hypothyroidism, you may notice weight gain, even if you aren't overeating. You may also be more sensitive to cold, have low energy, and have dry skin and hair. 

What other tests might I have along with this test?

You may have tests to measure T3 and TSH. These hormones also play key roles in thyroid health. You may also have a blood test to measure the level of certain antithyroid antibodies in your blood to get a more accurate diagnosis. 

What do my test results mean?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your healthcare provider.

The normal range for free T4 is 0.8 to 2.8 nanograms per deciliter (ng/dL). A level of free T4 that is higher than normal could mean you have an overactive thyroid. Conditions associated with hyperthyroidism include Graves' disease, an autoimmune disorder.

Abnormally low free T4 levels may signal hypothyroidism. This means your thyroid is not making enough hormones. An underlying condition, such as Hashimoto's disease, another autoimmune disorder, could be the cause of an underactive thyroid. 

How is this test done?

The test requires a blood sample, which is drawn through a needle from a vein in your arm.

Does this test pose any risks?

Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.

What might affect my test results?

Certain medicines, such as phenobarbital, can affect your free T4 levels. Severe chronic illnesses, such as chronic renal failure and cirrhosis of the liver, can also affect the test.

How do I get ready for this test?

You don't need to prepare for this test. But be sure your doctor knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use. 

Free T4 - WellSpan Health

Author: Vardigan, Benj
Online Medical Reviewer: Hanrahan, John, MD
Online Medical Reviewer: Taylor, Wanda, RN, Ph.D.
Last Review Date: 2015-07-09T00:00:00
Last Modified Date: 2015-08-14T00:00:00
Published Date: 2015-08-14T00:00:00
Last Review Date: 2012-05-10T00:00:00
© 2016 WellSpan Health. All Rights Reserved. This information is not intended as a substitute for professional medical care. Always follow your healthcare professional's instructions.

I would like to:

Are you sure you would like to cancel?

All information will be lost.

Yes No ×

About the provider search

This search will provide you with WellSpan Medical Group and Northern Lancaster County (Ephrata) Medical Group primary care physicians and specialists. If we don’t have a WellSpan Medical Group physician to meet your criteria, the search will expand to include community physicians who partner with WellSpan Medical Group physicians through the WellSpan Provider Network or provide care to patients on the Medical Staffs of WellSpan’s Hospitals.


Schedule Your Next Appointment Online with MyWellSpan

Use your MyWellSpan patient portal any time to view available appointments, and pick the date and time that best suits your schedule.

Go to MyWellSpan

New to this practice?

If you don't have a WellSpan primary care provider and would like to schedule a new patient appointment with a provider who is accepting patients, just log into your MyWellSpan account, and go to the Appointment Center section. As you progress through the scheduling process, you will be able to see the offices that are accepting new patients in relation to your zip code. If you are not enrolled in MyWellSpan, go to https://my.wellspan.org, call 1-866-638-1842 or speak with a member of the staff at a participating facility to sign up. New patient scheduling not available at all practices/programs.

Already a patient at this practice?

If you already have a relationship with a WellSpan practice, simply log into your account, and go to the Appointment Center section. As you progress through the scheduling process, you will be able to schedule an appointment with any provider or practice that already counts you as a patient. Online scheduling varies by practice/program.