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Thyroid health

Triiodothyronine

Also known as: Free T3 test, FT3, Triiodothyronine, free, Free triiodothyronine blood test

A free T3 test measures the amount of triiodothyronine (T3) circulating unbound in your blood. This is the active portion of the hormone that can enter tissues and affect metabolism.

Testing free T3 is usually not the first test. Providers typically start with TSH (and often Free T4). Free T3 can help when an overactive thyroid (hyperthyroidism) is suspected or to assess its severity, but according to the American Thyroid Association, free T3 testing is often less reliable and usually not helpful for most thyroid evaluations.

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Key takeaways

  • Sample required: Blood

  • Tests for: Levels of free (unbound) triiodothyronine (T3), the active thyroid hormone that regulates metabolism, heart rate, and body temperature

  • Healthy range: Typically 2.3–4.2 picograms per milliliter (pg/mL) in adults (ranges vary slightly by lab and assay)

What can a free T3 test tell you?

A free T3 test helps assess thyroid function and may:

  • Evaluate suspected hyperthyroidism when symptoms or other thyroid tests, like thyroid-stimulating hormone (TSH) or free thyroxine (T4), are abnormal

  • Clarify unclear or borderline results from other thyroid function tests in select cases 

The American Thyroid Association notes that T3 testing is helpful in diagnosing hyperthyroidism and assessing its severity, though it’s generally less informative in cases of hypothyroidism.

What is being tested? 

The test measures free triiodothyronine (T3) — the active form of thyroid hormone not bound to proteins in the blood.

T3 is produced mainly by the thyroid gland, though some is converted from T4 in other tissues like the liver and kidneys. It regulates how your body uses energy and affects processes such as:

  • Metabolism (how quickly your body burns calories)

  • Heart rate and cardiac output

  • Body temperature regulation

  • Digestive and muscle function

  • Brain function, mood, and alertness

  • Growth and bone health

Because it’s the metabolically active form, measuring free T3 (rather than total T3) theoretically provides a clearer picture of thyroid activity. However, because Free T3 assays vary, clinicians often use Total T3.

Where is the free T3 test typically included?

Free T3 is often part of a comprehensive thyroid panel, which may include:

  • Thyroid-stimulating hormone (TSH)

  • Free thyroxine (Free T4)

  • Total T3 or T4

This panel helps providers evaluate how the thyroid, pituitary, and hypothalamus interact to regulate hormone balance.

Who should get a free T3 test?

Your provider may order a free T3 test if you have symptoms suggesting an overactive thyroid or when another thyroid test shows abnormal results.

Common reasons for testing free T3 include:

  • Signs of hyperthyroidism such as weight loss, anxiety, heat intolerance, tremors, or rapid heart rate.

  • Abnormal or borderline TSH or T4 results.

  • Monitoring response to thyroid medications.

The American Thyroid Association recommends measuring thyroid function in all adults beginning at age 35, with more frequent screening in high-risk or symptomatic individuals.

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Not available in all 50 states. Eligibility and provider order required. Lab results alone are not intended to diagnose, treat, or cure any condition. A provider will reach out about critical results and plans include access to 24/7 provider messaging.

Images for illustrative purposes only.

Frequently asked questions

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What causes changes in free T3 levels?

A variety of factors can influence free T3 results.

Hyperthyroidism, Graves’ disease, thyroiditis (inflammation of the thyroid gland), pregnancy, and excessive thyroid hormone therapy are factors that may increase free T3 levels.

Conversely, factors that may lower free T3 levels include hypothyroidism, non-thyroidal illness syndrome and malnutrition, chronic fatigue syndrome, and use of beta-blockers.

What is a normal free T3 range?

Typically 2.3–4.2 picograms per milliliter (pg/mL) in adults (ranges vary slightly by lab and testing method).

Can this test be done at home?

Yes. Some at-home thyroid kits include free T3. However, you should always confirm results with a certified lab, since home tests may have greater variability.

Is fasting required?

Not usually, but you should always ask your provider for special instructions. Biotin should also be stopped at least 48 hours before testing.

How long do results take?

Results are typically available within 1–3 business days.

How often should I get tested?

There’s no set schedule for FT3 testing. Your provider may order it when symptoms, risk factors, or other thyroid tests point to a problem. 

The American Thyroid Association suggests that adults have their thyroid function checked starting at age 35 and every five years after that. People at higher risk or with symptoms may need testing more often. The U.S. Preventive Services Task Force (USPSTF) says there isn’t enough evidence to recommend for or against routine screening.

Does insurance typically cover it?

Most insurance plans cover thyroid testing when medically indicated, including free T3, TSH, and free T4. Always check your policy for specific details. Insurance is not required for lab testing through Hers. 

What tests are related to this biomarker?

• Thyroid-stimulating hormone

• Free thyroxine

• Total T3

• Thyroid antibodies (TPOAb, TgAb)

• Comprehensive thyroid panel

The information provided here is for informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always talk to a qualified healthcare provider about your specific health concerns.

Related biomarkers

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Thyroglobulin Antibodies (TgAb)

Thyroglobulin antibodies (TgAb) are immune system proteins that sometimes attack thyroglobulin, the material your thyroid needs to create hormones. Low levels mean your immune system is not interfering with your thyroid's ability to produce hormones.

Learn more about Thyroglobulin Antibodies (TgAb)

Thyroid Peroxidase Antibodies (TPOAb)

Thyroid peroxidase antibodies (TPOAb) sometimes attack thyroid peroxidase, an enzyme needed to produce thyroid hormones. Healthy, low levels mean your thyroid is free to make hormones without interference.

Learn more about Thyroid Peroxidase Antibodies (TPOAb)

Thyroid-Stimulating Hormone (TSH)

Thyroid-stimulating hormone (TSH) is the brain’s messenger to your thyroid that communicates the amount of thyroid hormone to make. It also drives metabolism, energy, and mood. A balanced level shows that your brain and thyroid are working together to keep your energy, metabolism, and mood steady.

Learn more about Thyroid-Stimulating Hormone (TSH)

Free Thyroxine (T4)

Free thyroxine (T4) is the main hormone made by your thyroid gland. While T4 itself has some effects in the body, its main job is to act as a "prohormone", a substance that your body converts into the active form, T3. Healthy levels mean your thyroid is making enough of the inactive hormone that your body can convert into active T3 when needed.

Learn more about Free Thyroxine (T4)

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Reviewed by Felix Gussone, MD

Published 12/04/2025