Test Quick Guide

Triiodothyronine, known as T3, is an important thyroid hormone that regulates metabolism and ensures your body can 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, while 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, assess the severity and type of thyroid disease, and 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 you have 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 used 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 help diagnose hyperthyroidism or a pituitary gland disorder, evaluate the cause of an overactive thyroid, and monitor your response to the treatment of hyperthyroidism.

What does the test measure?

T3 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 and 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, free T3, is not attached to proteins. Only the free T3 can act on cells and stimulate many physical processes. The body regulates the relationship between bound and free T3 in a 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 functions.

When should I get this test?

You will usually have a free or total T3 test when your doctor suspects you have a thyroid problem. This may be because you had an abnormal TSH test result or you are experiencing symptoms of an overactive thyroid, called hyperthyroidism, which 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 if you are hypothyroid.

Finding a T3 Test

How can I get a T3 test?

A doctor usually orders a free or total T3 test, 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.  However, you can also get tested through private labs or by ordering a test online.

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 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. This 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. They are also best positioned 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.

Taking a T3 Test

A blood sample measures 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.

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 can 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 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.

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?


See More

Ask a Laboratory Scientist

Ask A Laboratory Scientist

This form enables patients to ask specific questions about lab tests. Your questions will be answered by a laboratory scientist as part of a voluntary service provided by one of our partners, American Society for Clinical Laboratory Science. Please allow 2-3 business days for an email response from one of the volunteers on the Consumer Information Response Team.

Send Us Your Question