What Is a T3 Test?

Written by Marijke Vroomen Durning, RN
Medically Reviewed by Shruthi N, MD on July 24, 2025
7 min read

A T3 test measures the level of the thyroid hormone triiodothyronine (T3) in your blood. Your doctor uses it to check how well your thyroid works. It helps find conditions such as hyperthyroidism — when your thyroid is too active. Your doctor may also use it to see if your thyroid treatment works well.

Your thyroid is a small, butterfly-shaped gland under your larynx at the base of your neck. It releases two main hormones into your blood: triiodothyronine (T3) and tetraiodothyronine (T4). These hormones control how your body uses energy. Thyroid tests show if you have too much or too little of them.

Your doctor may also order other thyroid checks, such as a T4 test and a thyroid-stimulating hormone (TSH) test. If your TSH result is outside the normal range, your doctor may use a T3 test to confirm that your thyroid is the problem.

T3 makes up 10% of all hormones released by the thyroid. There are two forms of T3: free and bound. Most of the T3 in your blood is attached, or bound, to proteins, and only a small percentage of it is free. A total T3 test measures both bound and free T3.

A free T3 test measures the amount of free T3, the active hormone that your body can use as needed. The free T3 test is usually more helpful because it directly measures the hormone that is active in your body.

The other thyroid hormone is T4. As it moves through your blood, your body changes it into T3 when you need it. Most of this happens in your liver and kidneys.

 “T4 is like money in your bank account and T3 is like the money in your wallet,” says Joseph Barrera, MD, an endocrinologist at Providence Mission Hospital in Mission Viejo, California. T3 is what you use right away, and T4 is what you can draw on when you need more T3.

A T3 test is quite simple and doesn’t require any special preparation, such as overnight fasting. Unless your doctor says otherwise — perhaps you’re having other blood tests at the same time — you usually should eat as you would normally because fasting could affect your results. Research hasn’t shown a link between T3 levels and fasting, but fasting can influence TSH levels in your body.

Before you go for the test, you need to tell your doctor about any medications and nonprescription supplements you’re taking, as some can affect thyroid hormone levels. Your doctor will tell you if you need to stop taking them beforehand and, if so, how far in advance.

It is an ordinary blood test, where blood is taken from your arm using a small needle. It generally takes less than five minutes and has little risk involved. You may feel some pain or get some bruising after. The tenderness should go away within a few hours, but it could take a few days for the bruise to disappear.

Normal T3 levels

Normal levels of T3 vary depending on whether your doctor is looking at total T3 (both free and bound) or free T3 alone. It depends on age as well. Total T3 test results are typically measured in nanograms per deciliter (ng/dL), while free T3 is measured in picograms per deciliter (pg/dL).

T3 levels for healthy people are usually in the following ranges:

  • Children 1-5 years old: 106-203 ng/dL
  • Children 6-10 years old: 104-183 ng/dL
  • Children 11-14 years old: 68-186 ng/dL
  • Adolescents 15-17 years old: 71-175 ng/dL
  • Adults 18-99 years old: 79-165 ng/dL

Free T3 levels for healthy people usually fall in the following ranges:

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

High T3 levels

High T3 levels usually mean you have hyperthyroidism, or an overactive thyroid. This condition speeds up how your body uses energy. Symptoms may include:

  • Unexplained weight loss
  • Feeling shaky and nervous
  • Increased bowel movements increase
  • Fast or irregular heartbeat (arrhythmia)

Low T3 levels

Low T3 levels may mean hypothyroidism, or an underactive thyroid. Doctors don’t usually rely on T3 tests alone to diagnose it. That’s because some people with hypothyroidism can have a normal T3 level even when TSH is high and T4 is low.

Symptoms may include:

  • Feeling unusually tired and weak
  • Weight gain without changes in diet or activity
  • Dry skin and hair
  • Feeling cold, even in warm places
  • Slower heartbeat

How long do T3 test results take?

Facilities and clinics may have their own labs on-site, or they might send the blood to an outside lab for testing, so timing for results can vary. The usual time to wait for T3 results is one to two business days.

There are several reasons why someone may have abnormal T3 results.

Hypothyroidism

If you have hypothyroidism, your thyroid gland doesn’t make enough hormones. Low T3 could also be caused by:

  • Taking some types of medications
  • Having thyroid surgery
  • Getting radiation therapy for the head or neck area
  • Being pregnant or in the post-partum period (after delivering a baby)
  • Having an iodine deficiency
  • Having Hashimoto’s thyroiditis, an autoimmune disorder

Hyperthyroidism

Hyperthyroidism happens when your thyroid gland produces more thyroid hormones than you need. It could be caused by:

Graves’ disease. An autoimmune condition where your immune system causes your thyroid to work too hard.

Thyroiditis. Inflammation of your thyroid that can release too much thyroid hormone into your blood.

Thyroid nodules. Small growths on your thyroid that sometimes make extra hormone.

Too much iodine. Iodine is a mineral your thyroid uses to make hormones, and too much can cause an imbalance.

A growth on your pituitary gland (thyrotropinoma). A rare, noncancerous growth in your brain that triggers your thyroid to make too much hormone.

In rare cases, high T3 levels can be caused by thyrotoxicosis or thyroid cancer.

If you’re experiencing hyperthyroidism symptoms, your doctor might suggest you have a T3 test. “If your screening or TSH levels are normal but you still have symptoms of a thyroid disorder, talk to your doctor about free T3 testing,” Barrera says.

Other reasons to have a T3 test include:

  • Taking thyroid medication and needing follow-up tests to confirm the right dose
  • Having abnormal results on other thyroid tests
  • Showing signs of pituitary gland problems, which help control thyroid hormone levels

Some medications can affect the results of a T3 test. They include:‌

A TSH test measures how much of this hormone is in your blood. TSH is made by your pituitary gland, a small gland in your brain that tells your thyroid how much hormone to make. Your doctor may order this test to:

  • Diagnose thyroid disorders
  • Monitor your thyroid treatment
  • Screen for thyroid disease if you have risk factors
  • Check your thyroid before pregnancy if you’re planning to get pregnant

A T4 test measures how much of the thyroid hormone T4 is in your blood. A T3 test measures the amount of T3. These tests are usually done if your TSH results are abnormal or if you have symptoms of a thyroid problem and your doctor needs more information.

There are no risks to having a blood test to check your T3 levels. You might have some bruising or tenderness at the area where the blood was drawn, but these should go away within a few days.

A T3 test shows how much of the hormone T3 is in your body. You usually get it along with T4 and TSH tests, not by itself. This test helps diagnose hyperthyroidism, check if your thyroid medicine is working, or find out if there’s a problem with your pituitary gland. If you notice signs of hyperthyroidism or have concerns about your thyroid, talk to your doctor about a T3 test.

When should T3 levels be checked?

Your doctor will decide if you need to have your T3 levels checked. It might be done if you show signs of hyperthyroidism or if you’re already receiving medication to manage a thyroid problem.

Does T3 help with weight loss? 

T3 should not be used to help lose weight. It is a hormone given in very precise doses that can cause serious or life-threatening issues if not used as directed.

Are there any drugs that affect thyroid function tests?

There are several medications and even some supplements that can affect the results of thyroid function tests. It’s important to tell your doctor about all drugs you take, including prescription, over-the-counter, supplemental, and recreational drugs, before having a thyroid function test.

Why is T3 not routinely tested?

T3 isn’t a routine test because the TSH test is more sensitive and tells your doctor more than just the T3 alone. A T3 test is usually done only if your doctor suspects hyperthyroidism or wants to check how well your thyroid medications are working.