This Free Blood Test is Key to Helping Identify Thyroid Disorder

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

What are normal and abnormal free T3 levels? What can you 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 bloodstream. The thyroid gland produces triiodothyronine and an additional hormone called thyroxine (T4).

It is crucial to note T3 blood tests are notoriously variable, if sometimes unreliable.

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

A healthcare provider can order additional thyroid tests, including TSH and T4, to evaluate thyroid function. 

What Does a T3 Do?

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

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

T3 is the functional form of the thyroid hormone. But T4 is more stable than T3. Therefore, the body only converts T4 to T3 when necessary. 

T4 and T3 manage how the body uses and then stores energy for metabolism. It also aids the following body functions: Cholesterol, breathing, body weight, body temperature, the nervous system, and muscle strength.

Reasons you might need a free T3 Blood Test

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

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

Symptoms prompting a healthcare provider to order a T3 blood test include weight loss, anxiety, bulging eyes, anxiety, difficulty sleeping, hand tremors, heat intolerance, fatigue, and frequent bowel movements. 

How do you prepare for a free T3 blood test?

You may need to discontinue certain medications before having a free T3 blood test. A healthcare provider will provide information, but medicine should only be stopped if instructed.

The following medications can alter T3 blood tests and can need to be stopped: Estrogen, birth control pills, biotin, propranolol, methadone, anabolic steroids, lithium, amiodarone, anti-thyroid medications, androgens, and phenytoin.

Procedure for a T3 blood test

A T3 blood test can be done in a lab, hospital, or medical provider’s office. A blood sample is taken and will be processed in a lab. Test results are reported back after a few business days. Ask your healthcare provider when you should expect your test results.

What are normal T3 levels?

Normal free T3 levels will vary based on an individual’s age. Here is the breakdown based on age: O 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; and 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 deciliters (dL) and nanograms (ng). Anytime there is a question about a blood result, talk to your healthcare provider to help interpret the results. 

What do abnormal T3 test results indicate?

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

High T3 levels

Abnormally high free T3 levels indicate that someone may have toxic nodular goiter, hyperthyroidism, T3 thyrotoxicosis, liver disease, or toxic nodular goiter.

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

Low T3 levels

Low free T3 levels could indicate thyroiditis, starvation, or an underactive thyroid gland. Some severe illnesses can also cause low T3 levels. 

T3 blood test risks and side effects

A T3 blood test’s risks are similar to other blood tests. There are a few mild side effects that some people can experience. These are infection, needing multiple sticks to find the vein, bruising at the needle insertion site, and feeling lightheaded. 


A free T3 blood test is used along with other tests to diagnose thyroid disease, typically hyperthyroidism. The blood test is done similarly to other blood tests, with a blood sample from a vein. 

Results are usually ready within a few days and can provide healthcare providers with the information necessary or may require additional testing to determine a correct diagnosis.