T3 Blood Test (Free T3): What It Is, Function, Levels, and More

A free T3 blood test, also known as a free triiodothyronine (T3) test, measures the amount of triiodothyronine in the blood. This hormone is made by the thyroid gland, which is responsible for many body functions. The blood test will diagnose certain thyroid conditions like hyperthyroidism.

This article will discuss normal and abnormal free T3 levels. It will also cover what to expect when receiving a free T3 test.

What Is a T3 Blood Test?

A T3 blood test measures the level of triiodothyronine hormone in the blood. The thyroid gland makes triiodothyronine and another hormone called thyroxine (T4).

It is important to note that T3 blood tests are notoriously variable, even sometimes unreliable.

There are three types of T3 blood tests. Free T3 measures triiodothyronine that is not attached to anything and freely moves in the blood. Bound T3 measures triiodothyronine that is attached to protein. The last test is a total T3 test, which measures free and bound T3.

A healthcare provider may order other thyroid tests, including T4 and TSH to evaluate thyroid function.

What Does a T3 (Triiodothyronine) Do?

The thyroid is a butterfly-shaped gland in the front of the neck above the collarbone. It makes thyroid hormones, which are used for various body functions.

The thyroid primarily produces the hormone thyroxine (T4). The T4 comes from the four iodine atoms it contains. T4 is changed to triiodothyronine (T3) by removing one of the iodine atoms. This is done in the liver and other tissues.

T3 is the functional form of the thyroid hormone. However, T4 is more stable than T3. Therefore, the body will only convert T4 to T3 as needed.

T3 and T4 work to manage how the body uses and stores energy for metabolism. It also aids in the following body functions:

Reasons You Might Need a Free T3 Blood Test

A T3 blood test may be done to check for thyroid disease, typically to help determine if someone has hyperthyroidism. However, it is T4 levels that are more commonly tested to evaluate for a hyperthyroid or hypothyroid condition.

T3 blood tests can also be done to monitor how well thyroid treatment is working.

Symptoms that may prompt a healthcare provider to order a T3 blood test include:

• Bulging eyes
• Weight loss
• Difficulty sleeping
• Anxiety
• Fatigue
• Heat intolerance
• Frequent bowel movements
• Hand tremors

How to Prepare for a Free T3 Blood Test

You may need to stop certain medications before having a free T3 blood test. A healthcare provider will provide that information; no medicine should be stopped unless instructed.

The following medications can alter T3 blood tests and may need to be stopped:

• Birth control pills
• Biotin
• Estrogen
• Methadone
• Amiodarone
• Propranolol
• Lithium
• Anabolic steroids
• Androgens
• Phenytoin
• Anti-thyroid medications

The T3 Blood Test Procedure

A T3 blood test is typically done in a medical provider’s office, lab, or hospital. A healthcare provider will take a blood sample. This process involves cleaning the skin, inserting a needle into a vein, and drawing the blood out through the needle into the tubing. A bandage will be placed over the insertion site. You may experience a small amount of bruising and pain.

The blood sample will be processed in a lab. Test results are reported back in a few business days. However, each location’s timing will vary. Be sure to ask the healthcare provider performing the procedure when you should expect your results.

What Are Normal T3 Levels?

Normal free T3 levels will vary based on a person’s age. Below is the breakdown based on age:

• 0 to 3 days: 2.0–7.9pg/mL
• 4 to 30 days: 2.0–5.2pg/mL
• 31 days to 12 months: 1.6–6.4pg/mL
• 13 months to 5 years: 2.0–6.0pg/mL
• 6 to 10 years: 2.7–5.2pg/mL
• 11 to 19 years: 2.3–5.0pg/mL
• Older than 19 years: 2.0–4.4pg/mL

The above values are based on picograms per milliliter (pg/mL). Values can also be given in nanograms (ng) and deciliters (dL). Anytime there is a blood result question, talk to a healthcare provider to help interpret the results.

What Do Abnormal T3 Test Results Mean?

An abnormal T3 test could indicate there is something wrong with the thyroid gland. Results will be compared with other thyroid tests like T4 and TSH.

High T3 Levels

Abnormally high free T3 levels indicate that someone may have:

High T3 levels could also result from someone taking thyroid medication, birth control pills, or estrogen. T3 is also high during pregnancy, especially at the end of the first trimester.

Low T3 Levels

Low free T3 levels could indicate the following:

Some severe illnesses can also cause low T3 levels.

T3 Blood Test Risks and Side Effects

The risks of a T3 blood test are similar to other blood tests. There are a few mild side effects that some people may experience. These are:

• Bruising at the needle insertion site
• Infection
• Feeling lightheaded
• Needing multiple sticks to find the vein

Summary

A free T3 blood test is used in conjunction with other tests to diagnose thyroid disease, typically hyperthyroidism. The blood test is done similarly to other blood tests, with a blood sample taken from a vein. Results will be ready within a few days and may provide healthcare providers with the information needed or may require additional testing to determine a diagnosis.