What Is a Triiodothyronine Test?

A free triiodothyronine test, most commonly referred to as a T3 test, measures the level of a hormone called triiodothyronine in your blood. T3 is produced by your thyroid in response to thyroid stimulating hormone (TSH), which is created by the pituitary gland. T3 is also created when thyroxine (T4), the main thyroid hormone, is chemically converted to T3. (There is also reverse T3, an inactive form of T3 that has some conflicting research surrounding it, but it is not part of a standard T3 test.)

Together, T3 and T4 help control your body’s metabolism—how it uses and stores energy. It can also help regulate your cholesterol level, heart function, nervous system, brain development, and body temperature, among other processes.

Purpose of a T3 Test

There are two types of T3—bound T3, which is attached to proteins to transport the hormone throughout the body, and free T3, which circulates through the bloodstream unattached. A T3 test will measure both bound and free amounts of T3 in the blood.

You may need a T3 test if you are showing signs and symptoms of hyperthyroidism, including mood swings and irritability, sleeping problems, sweating, low tolerance to heat, unexplained weight loss, menstrual irregularity, high blood sugar, shortness of breath, and hair loss. If you are 60 years old or above with a family history of thyroid disease or have type 1 diabetes, your healthcare provider may also order a T3 test to check for hyperthyroidism.

A T3 test can also help diagnose hypothyroidism. If you’re showing signs and symptoms of this condition, such as extreme fatigue, low tolerance to cold, unexplained weight gain, hair loss, shortness of breath, or menstrual irregularity, your practitioner may want to run a T3 test to understand how your thyroid is functioning.

In addition to a T3 test, your practitioner may also recommend a T4 test, a TSH test, thyroid scan, radioactive iodine uptake test, or a thyroid-stimulating immunoglobin test. Most commonly, T3 is tested when you have an abnormal TSH level with a normal T4 range to help determine whether you may have thyroid disease or an issue with your pituitary gland.

A total T3 test will measure both bound and free T3 in the blood, while a free T3 test only measures unbound T3. Usually it is sufficient to measure total T3, unless it is suspected that a patient has an alteration in binding proteins, such as during pregnancy. In those cases, free T3 may be measured instead.

Risks and Contraindications

A T3 is done with a blood draw, and there are usually no risks with the procedure aside from the risks that may occur with getting a blood sample taken. You may have a headache or feel slightly queasy during the draw, or notice tenderness, redness, or slight bruising at the site where the draw took place. If you feel nervous about having blood taken, make sure to tell the technician before they start the test, so they can be aware and help make you comfortable during and after the test.

Eating a small snack and drinking something like juice or water after the blood draw may help with light-headedness and any anxiety you may have associated with needles.

The Test

Before the Test

Give your healthcare provider a detailed list of all medications, vitamins, supplements, and herbs you are taking before getting a T3 test. Your practitioner will let you know if there’s any medication you should stop taking prior to the test (or they may have you wait to take medication until after the blood draw). Certain drugs and supplements can increase T3 levels, including birth control, estrogens, methadone, clofibrate, biotin, and some herbs.

Drugs that can decrease T3 levels include amiodarone, anabolic steroids, androgens, antithyroid drugs, lithium, phenytoin, and propranolol. The test itself should only take a few minutes, and the results should be available from a few days up to a week after your practitioner sends your blood to an outside lab to be examined (if, in fact, the facility in which you get blood drawn does not produce the results).

Location

A T3 test will likely take place right in your medical professional’s office. In certain cases, you may have to go to a separate lab to get your blood drawn at a time that is convenient for you.

What to Wear

Remember to wear a shirt that can be easily rolled up so that you can remove it for the blood draw. It’s important for the technician performing the blood draw to be able to access your arm to find the best vein to take blood from with minimal discomfort to you.

Food and Drink

Unless specified by your healthcare provider, hold off on any vitamins or medications until after your free T4 test. Make sure you eat a healthy meal or snack an hour or two before your free T4 test to prevent getting dizzy or lightheaded during the blood draw.

Cost and Health Insurance

When medically necessary, a T3 test is usually covered by health insurance. Depending on your plan coverage, you may have to pay a coinsurance fee.

Interpreting Results

T3 results are rarely looked at in isolation. While it is helpful in determining the severity of hyperthyroidism (those with hyperthyroidism will have a higher T3 level), T3 levels alone are not often used in diagnosing hypothyroidism, because it’s the last hormone level to show abnormalities. It also changes constantly throughout the day.

Normal levels of T3 in an average adult are 60 to 180 nanograms per deciliter (ng/dL) for total T3 and 130 to 450 picograms per deciliter (pg/dL) for free T3. There are many factors that contribute to T3 levels, including age, pregnancy (which can temporarily cause a thyroid issue and higher T3 levels), and previous health history.

A Word From Verywell

Abnormal T3 levels are just one piece of the puzzle when it comes to your thyroid health. While you may feel alarmed that your healthcare provider has suggested one for you, it’s a common test that many of your friends and family have possibly already done themselves. Running a T3 test is simply a diagnostic step that will help your practitioner investigate your thyroid health.

If you’re already in the process of thyroid treatment, your practitioner will frequently test your T3, T4, and TSH levels to make sure the treatment is effective and determine whether any medication needs to be adjusted. As with any medical test, it’s important to discuss the results with your medical professional first to make sure you understand the results and how they impact your thyroid.