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  • Also Known As:
  • Free Triiodothyronine
  • Total Triiodothyronine
  • T3 Radioimmunoassay
  • FT3
  • TT3
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Test Quick Guide

T3 is an important thyroid hormone that regulates metabolism and ensures your body is able to perform essential functions properly. While most T3 in the bloodstream is bound to protein, some is unbound, or free. The free T3 test measures the active form of T3 that is not bound to protein. The total T3 test measures bound T3 and free T3 combined.

Free T3 or total T3 blood tests may be ordered to evaluate thyroid function if a thyroid disorder is suspected. They may also be used to evaluate pituitary gland problems, to assess the severity and type of thyroid disease, and to monitor treatment for a thyroid condition.

About the Test

Purpose of the test

Both free T3 and total T3 testing are used to evaluate thyroid function. Along with other thyroid function tests, free and total T3 tests play a role in diagnosing thyroid disorders, such as an overactive or underactive thyroid gland, assessing the severity of thyroid problems, diagnosing pituitary gland disorders, and monitoring patients with known thyroid conditions.

T3 testing may be used as a follow-up test if a patient has had an abnormal thyroid-stimulating hormone (TSH) test.

A free or total T3 test is often part of a thyroid panel, a group of tests which are used together to evaluate thyroid health.

Either test may be used to evaluate thyroid function:

  • Free T3 testing may be used to determine whether thyroid replacement therapy is effective. It can also help diagnose abnormalities of the proteins that bind thyroid hormones.
  • Total T3 is considered by many experts to be a more reliable measurement than free T3. Total T3 may be used to help diagnose hyperthyroidism or a pituitary gland disorder, evaluate the cause of an overactive thyroid, and monitor a patient’s response to treatment of hyperthyroidism.

What does the test measure?

T3, also called triiodothyronine, is a thyroid hormone that plays a major role in the body. Along with T4, another important thyroid hormone, T3 regulates the body’s metabolism, activates cells, and enables vital organs to function.

Triiodothyronine is referred to as T3 because this hormone has three iodine atoms attached to it. The other major thyroid hormone, thyroxine or T4, has four iodine atoms. Some of the T3 circulating in your body is made in the thyroid. Most of it, however, starts as T4 then is converted in the blood into T3 by the removal of an iodine atom.

More than 99% of the T3 in the body is bound to proteins as it circulates in the bloodstream. A small proportion, known as free T3, is not attached to proteins. Only the free T3 is able to act on cells and stimulate many physical processes. The relationship between bound and free T3 is regulated by the body in a very fine balance to ensure proper body functions.

The total T3 test measures both bound and free T3, while the free T3 test measures only the T3 that is not attached to protein.

T3 measurements are typically used along with other thyroid function tests, like TSH and free and total T4, to evaluate how your thyroid is functioning.

When should I get a T3 test?

People will usually have a free or total T3 test when their doctor suspects they have a thyroid problem. This may be because they had an abnormal TSH test result, or it may be because they are experiencing symptoms of an overactive thyroid, called hyperthyroidism. Symptoms of hyperthyroidism may include:

  • Weight changes, usually weight loss
  • Fatigue
  • Frequent bowel movements
  • Hair loss
  • Increased appetite
  • Increased sweating
  • Menstrual irregularities
  • Heart palpitations
  • Insomnia
  • Nervousness or anxiety
  • Problems with concentration

Although it may be used sometimes, T3 is less helpful in diagnosing hypothyroidism, because T3 is typically the last hormone to become abnormal in hypothyroid patients.

Finding a T3 Test

How to get tested

A free or total T3 test is usually ordered by a doctor, often as part of a panel or group of thyroid tests. Testing is typically performed in a medical setting like a doctor’s office, hospital, clinic, or medical laboratory.

Can I take the test at home?

It is possible to test your free T3 using an at-home test, usually as part of a group of tests.

Most commercially available at-home thyroid test kits are thyroid panels, meaning several thyroid tests are performed on a single blood sample. Which thyroid function tests are included in the thyroid panel may vary slightly from kit to kit.

The commercially available thyroid function test kits require you to provide a blood sample using a finger prick. The sample is then mailed to a lab for testing to be analyzed. After your sample is received and analyzed, your test results will be available via a secure online platform.

Your doctor can help you weigh the pros and cons of at-home thyroid testing for your specific situation. Your doctor is also in the best position to interpret results from an at-home test

How much does the test cost?

The cost of a free or total T3 test will vary depending on factors like where the test is done, whether it is part of a panel of tests, and whether you have health insurance. If your doctor orders a free or total T3 test, insurance typically covers its cost, although you may have to pay a copay or deductible. Your doctor’s office, lab, and health plan can tell you about any out-of-pocket costs that may be your responsibility.

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Taking a T3 Test

A blood sample is used to measure your free or total T3. The sample will usually be taken from your arm in a medical setting like a physician’s office or hospital and analyzed in a medical laboratory.

Before the test

A number of medications can affect your free or total T3 test results. These include:

  • Birth control pills
  • Female hormones such as estrogen
  • Methadone
  • Some herbs or supplements
  • Steroids
  • Male hormones, or androgens
  • Lithium
  • Propranolol

It is important to ask your doctor if you need to discontinue any medications prior to your test. Do not stop taking any medication without discussing it first with your doctor.

During the test

Most often, a blood sample is taken from a vein in your arm. The person taking the sample may tie a band around your upper arm and will disinfect a small patch of skin. Next, they will insert a needle attached to a collection tube into the disinfected area and withdraw a small amount of blood.

The process of taking a blood sample typically takes no more than three minutes.

After the test

After your blood sample has been removed and the needle is withdrawn, you will be asked to apply gentle pressure to the site with a bandage or a piece of gauze. This stops the site from bleeding and may prevent bruising. Next, a bandage will be applied. You will be able to resume your normal activities following the test.

There are limited risks associated with a blood draw. You may have slight bruising at the site where the sample was extracted.

T3 Test Results

Receiving test results

The doctor who ordered your free or total T3 test may share your test results with you, or you may be able to access them through an online portal. Free and total T3 results are usually available within a few business days. Results may take longer when a T3 test is performed as part of a thyroid panel that includes tests performed at a specialty lab.

Interpreting test results

Your free T3 or total T3 test will indicate whether your T3 is within the reference range, below normal, or higher than normal.

Reference ranges may vary slightly from laboratory to laboratory. The American Board of Internal Medicine uses the following reference ranges:

  • Free T3: 2.3–4.2 picograms per milliliter (pg/mL)
  • Total T3: 80–180 nanograms per deciliter (ng/dL)

Your doctor will interpret your free or total T3 test results together with other tests, such as thyroid-stimulating hormone (TSH) and total and free T4. The table below provides examples of test results and their possible interpretation:

Subclinical hypothyroidism and hyperthyroidism are conditions defined by abnormal TSH test results with normal T3 and T4 levels. These subclinical conditions often cause no symptoms or only very mild ones. Some subclinical thyroid conditions may progress to outright hypothyroidism or hyperthyroidism over time.

Are test results accurate?

Some expert groups consider total T3 to be a more reliable test than free T3 especially for the assessment of hypothyroidism. This is because a number of factors can impact the free T3 level in the blood, including:

  • Pregnancy
  • Disease
  • Medications

Additionally, there can be significant variation between different laboratory methods used to measure free T3, so the same blood sample may produce a different result depending on which method is used.

Do I need follow-up tests?

If your total or free T3 is higher than normal, you will most likely have additional tests to reveal why your thyroid gland is overactive. These may include:

  • Thyrotropin receptor antibody (TRab) testing is a blood test to confirm or rule out a diagnosis of Graves disease
  • A thyroid scan is an imaging test that will reveal whether you have a enlarged or inflamed thyroid gland
  • Radioactive iodine uptake is an imaging test that measures the rate your thyroid gland absorbs radioactive iodine in a certain time period.

Imaging tests such as the thyroid scan and the radioactive iodine uptake test reveal whether the entire thyroid is affected or whether only certain areas, called nodules, are abnormal.

Questions for your doctor about test results

Your doctor is in the best position to discuss your T3 test results with you. You may wish to ask your doctor specific questions such as:

  • Was my free or total T3 test normal?
  • Will additional tests be necessary to diagnose or rule out thyroid disease?
  • Do you recommend treatment to correct any thyroid abnormalities detected?

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