What Is a Triiodothyronine Test?

Understanding Your Thyroid Function

In This Article

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 that is attached to proteins in the body to transport the hormone throughout, 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 doctor may also order a T3 test to test for hyperthyroidism.

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

In addition to a T3 test, your doctor may also recommend a T4 test, a TSH test, thyroid scan, radioactive iodine uptake test, or a thyroid-stimulating immunoglobin (TSI) 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 and 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 lightheadedness and any anxiety you may have that is associated with needles.

The Test

Before the Test

Give your doctor a detailed list of all medications, vitamins, supplements, and herbs you are taking before getting a T3 test. Your doctor 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 can increase T3 levels, including birth control, estrogens, methadone, clofibrate, 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 doctor 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 doctor’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 doctor, 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, anywhere from 10 to 50 percent.

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.

In cases of hypothyroidism, it’s best to have a comprehensive thyroid evaluation with T4 and TSH tests to get a full picture of how your thyroid is functioning.

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 doctor 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 prompt your doctor to run more tests to investigate any abnormal levels your T3 test may show.

If you’re already in the process of thyroid treatment, your doctor will frequently test your T3, T4, and TSH levels to make sure the treatment is effective, and whether or not any medication needs to be altered. As with any medical test, it’s important to discuss the results with your doctor first to make sure you understand the results and how they impact your thyroid. 

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