T3 (Triiodothyronine) Test

What is a T3 (triiodothyronine) test?

A T3 (triiodothyronine) test helps diagnose thyroid conditions, particularly hyperthyroidism (overactive thyroid).

Your thyroid is a small, butterfly-shaped gland located at the front of your neck under your skin. It’s a part of your endocrine system. Triiodothyronine, also known as T3, is one of the two main thyroid hormones. Thyroxine, or T4, is the other hormone.

Healthcare providers test T3 levels using blood tests. Triiodothyronine comes in two forms:

• Free T3: This form enters your body’s tissues where it's needed.
• Bound T3: This form attaches to proteins, which prevents it from entering your body’s tissues.

Because of this, there are a few different tests that measure T3 levels. A blood test that measures both free T3 and bound T3 is called a total T3 test. A different blood test measures just free T3 levels. The tests for free T3 are generally less accurate than for total T3.

Healthcare providers often order additional tests to assess thyroid function alongside a T3 test, including a T4 (thyroxine) test and a TSH (thyroid-stimulating hormone) test.

Other names for a T3 test include:

• Thyroid function test.
• Total triiodothyronine.
• Free triiodothyronine.
• FT3.

What is a T3 (triiodothyronine) and what does it do?

Triiodothyronine, also known as T3, is one of the two main hormones your thyroid gland releases into your bloodstream. Your thyroid also produces thyroxine, also known as T4 and tetraiodothyronine. T4 and T3 work together and are commonly referred to as “thyroid hormone.”

Most of the T3 (approximately 80%) in your blood is from your body’s conversion of T4 into T3 outside of your thyroid gland. The rest of the T3 in your bloodstream is produced by your thyroid gland.

Hormones are chemicals that coordinate different functions in your body by carrying messages through your blood to your organs, muscles and other tissues. These signals tell your body what to do and when to do it.

T3 is the active form of thyroid hormone, meaning it impacts cells in your body, whereas T4 is the inactive form of thyroid hormone. Your liver and kidneys convert most of the T4 your thyroid releases into T3.

Together, T4 and T3 play vital roles in regulating your body’s:

• Metabolic rate (the rate at which your body transforms the food you eat into energy).
• Heart and digestive functions.
• Muscle control.
• Brain development.
• Bone maintenance.

Why do I need a T3 (triiodothyronine) test?

Healthcare providers most often order T3 tests to help diagnose hyperthyroidism, a condition in which your thyroid makes too much thyroid hormone, or to determine the severity of hyperthyroidism.

Your provider may order regularT3 tests to monitor your T3 levels if you’re taking thyroid hormone replacement therapy (medication) for a thyroid condition.

Who performs a T3 (triiodothyronine) test?

A healthcare provider called a phlebotomist usually performs blood draws, including those for a T3 blood test, but any healthcare provider who is trained in drawing blood can perform this task. They then send the samples to a lab where a medical laboratory scientist prepares the samples and performs the tests on machines known as analyzers.

How do I prepare for a T3 (triiodothyronine) test?

You usually don’t need to do anything special for a T3 blood test. Depending on the reason for the test, you may need to stop taking certain medications or supplements. In any case, your healthcare provider will give you specific instructions.

What should I expect during my T3 (triiodothyronine) blood test?

You can expect to experience the following during a blood test, or blood draw:

• You’ll sit in a chair, and a healthcare provider will check your arms for an easily accessible vein. This is usually in the inner part of your arm on the other side of your elbow.
• Once they’ve located a vein, they’ll clean and disinfect the area.
• They’ll then insert a small needle into your vein to take a blood sample. This may feel like a small pinch.
• After they insert the needle, a small amount of blood will collect in a test tube.
• Once they have enough blood to test, they’ll remove the needle and hold a cotton ball or gauze on the site to stop the bleeding.
• They’ll place a bandage over the site, and you’ll be finished.

The entire procedure usually takes less than five minutes.

What should I expect after my T3 (triiodothyronine) test?

After a healthcare provider has collected your blood sample, they’ll send it to a laboratory for testing. Once the test results are back, your provider will share the results with you.

What are the risks of a T3 (triiodothyronine) blood test?

Blood tests are a very common and essential part of medical testing and screening. There’s very little risk to having a T3 blood test. You may have slight tenderness or a bruise at the site of the blood draw, but this usually resolves quickly.

When should I know the results of my T3 (triiodothyronine) test?

In most cases, you should have your T4 test results within one or two business days, though it could take longer.

What type of results do you get for a T3 (triiodothyronine) test?

Blood test reports, including T3 test reports, usually provide the following information:

• The name of the blood test or what was measured in your blood.
• The number or measurement of your blood test result.
• The normal measurement range for that test.
• Information that indicates if your result is normal or abnormal or high or low.

What are normal T3 levels?

Normal value ranges for any lab test, including T3 (triiodothyronine) tests, may vary slightly among different laboratories. Be sure to check your lab report’s reference range on your results. If you have any questions about your results, ask your healthcare provider.

Normal T3 levels

Normal T3 level ranges vary based on age. In general, normal ranges for T3 for healthy people include:

• Children 1 to 5 years old: 106 – 203 nanograms per deciliter (ng/dL).
• Children 6 to 10 years old: 104 – 183 ng/dL.
• Children 11 to 14 years old: 68 – 186 ng/dL.
• Adolescents 15 to 17 years old: 71 – 175 ng/dL.
• Adults 18 to 99 years old: 79 – 165 ng/dL.

Normal free T3 levels

Providers don’t usually order free T3 tests because they’re not as reliable, but it is possible to test these levels. In general, normal ranges of free T3 for healthy people include:

• Infants up to 3 days old: 1.4 – 5.4 picograms per milliliter (pg/mL).
• Infants 4 to 30 days old: 2.0 – 5.2 pg/mL.
• Babies 1 month to 1 year old: 1.5 – 6.4 pg/mL.
• Children 1 to 6 years old: 2.0 – 6.0 pg/mL.
• Children 7 to 11 years old: 2.7 – 5.2 pg/mL.
• Children 12 to 17 years old: 2.3 – 5.0 pg/mL.
• Adults 18 to 99 years old: 2.3 – 4.1 pg/mL.

What happens when T3 levels are high?

Higher-than-normal T3 levels typically indicate hyperthyroidism (overactive thyroid). Hyperthyroidism has several causes, including Graves’ disease (an autoimmune condition), thyroid nodules and thyroiditis (inflammation of your thyroid gland).

Hyperthyroidism speeds up your metabolism, which can be dangerous to your health. Some symptoms of hyperthyroidism include:

• Unexplained weight loss.
• Feeling shaky and/or nervous.
• Increased bowel movements.
• Rapid or irregular heartbeat (arrhythmia).

If you’re experiencing symptoms of hyperthyroidism, it’s important to contact your healthcare provider.

If you’ve already been diagnosed with hyperthyroidism, T3 tests can help determine how severe it is. In general, the more elevated your T3 levels, the more severe the hyperthyroidism is.

What happens when T3 levels are low?

Lower-than-normal T3 levels may indicate you have hypothyroidism (underactive thyroid). However, healthcare providers don’t typically rely on T3 tests to diagnose hypothyroidism because it’s usually the last of the thyroid function tests to come back abnormal.

In addition, some people can have severe hypothyroidism with a high TSH level and a low free T4 level but have a normal T3 level.

Lower-than-normal T3 levels can also be due to medications like steroids and amiodarone (arrhythmia medication) and severe illness. These factors can decrease the amount of T4 (inactive hormone) your body converts into T3 (active hormone), resulting in a lower level of T3.

Should I be concerned if I have a low or high T3 (triiodothyronine) test result?

Total T3 test results are usually accurate. However, certain factors may interfere with the results, including certain medications or supplements and pregnancy. Your healthcare provider will consider these factors when interpreting your results.

A note from Cleveland Clinic

Seeing an abnormal test result can be stressful. Know that thyroid conditions are somewhat common and treatable. Your healthcare provider will let you know if you need to undergo further tests to determine the cause of the abnormal T3 level. Don’t be afraid to ask your provider questions. They’re there to help you.