What Is a T3 Test For?

What Is the Thyroid?

The T3 test measures triiodothyronine in blood to help doctors diagnose hyperthyroidism.
The T3 test measures triiodothyronine in blood to help doctors diagnose hyperthyroidism.

The thyroid is a butterfly-shaped gland located at the base of your throat. Its job is to make and release hormones that circulate through your body via your blood.

The hormones your thyroid makes help control how your body uses energy. Specifically, they turn your metabolism up or down. 

Thyroid problems can cause a wide range of symptoms and affect your: 

  • Appetite and weight
  • Body temperature
  • Energy level
  • Heart rate

If your doctor thinks something is off with your thyroid, he may order a T3 test. This test measures the amount of a specific thyroid hormone in your blood. 

What Is T3?

When your thyroid gland is working properly, it makes and releases a hormone called thyroxine, or T4. 

Your blood carries T4 around your body where your liver, brain, and other tissues turn some of it into triiodothyronine, or T3. 

Experts don’t understand exactly how T3 and T4 do what they do. But it’s clear that too much or too little of either hormone can throw off your metabolism and cause problems.

What Is the T3 Test For?

A T3 test measures the amount of T3 hormone in your blood.

What’s the Purpose of the T3 Test?

Hyperthyroidism causes a fast heartbeat
Hyperthyroidism causes a fast heartbeat.

If you have symptoms that might point to thyroid problems, this test helps your doctor identify the cause. 

Specifically, the T3 test can help doctors diagnose hyperthyroidism. This is a condition in which your thyroid makes too much T4 hormone. 

Hyperthyroidism Symptoms

Hyperthyroidism causes symptoms like: 

Doctors don’t usually order the T3 test if they suspect hypothyroidism, or an underactive thyroid. Other tests can better spot this thyroid issue. 

The T3 test can also help your doctor identify thyroid cancer and other uncommon conditions that can affect your thyroid.

What Are Normal, High, and Low Levels Test Results Mean?

  • Normal T3 levels in the blood range from 100 to 200 ng/dL (nanograms per deciliter). 
  • If your levels are higher than normal, that could mean you have an overactive thyroid (hyperthyroidism) or an uncommon condition such as thyroid cancer.
  • If your levels are lower than normal, that could be a sign of an underactive thyroid (hypothyroidism) or chronic illness. Your doctor will need to order other tests in order to diagnose these issues.

What’s the T3 Testing Procedure?

A nurse or clinician will draw blood from a vein in your arm or elsewhere.

Preparing for the T3 Test

You can eat and drink as usual. But, tell your doctor if you’re taking any medications. That includes prescription drugs, over-the-counter medicines, and supplements, including multivitamins. These can throw off your T3 test results. 

Also, tell your doctor if you’ve been under the weather. A cold or other short-term illness can lead to incorrect test results.

Total T3 vs. Free T3

  • In most cases, the T3 test your doctor orders will measure “total” T3. This is the amount of T3 hormone in your blood.
  • There is a second T3 test -- known as the “free T3” test -- that looks for only certain types of T3 hormone in your blood. But, doctors usually don’t order this test because the results aren’t as helpful.

Other Important Stuff to Know About the T3 Test and Other Thyroid Tests

Doctors often order the T3 test along with other blood tests that look at thyroid function, such as the TSH test. They do this because the T3 test alone may not help them diagnose or assess your health status.

Pregnancy, liver disease, and some other common conditions or illnesses can throw off the results of your T3 test.

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American Thyroid Association: “What You Need to Know About the Thyroid,” “Thyroid Function Tests.”

InformedHealth.org: “How Does the Thyroid Gland Work?”

UCLA Health: “T3 (triiodothyronine) Test.”

Mayo Clinic: “Hyperthyroidism (overactive thyroid): Symptoms & Causes.”

Lab Tests Online: “T3 (Free and Total).”