Understand Free T3 (Free Triiodothyronine)

Free T3 (Free Triiodothyronine)

Hormone markers

What the test generally measures, and what to ask about it.

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Updated Jul 13, 2026

What is Free T3 (Free Triiodothyronine)?

The active, unbound portion of the thyroid hormone T3 that is free to enter your tissues.

T3 (triiodothyronine) is one of the two hormones your thyroid makes, and free T3 is the active portion not bound to proteins. It is used most often to help confirm and understand an overactive thyroid, interpreted together with TSH and free T4 rather than alone. Higher free T3 can be associated with hyperthyroidism. Note that free T3 assays are often less reliable than a total T3 measure, so free T3 is used selectively and total T3 is frequently preferred; a result only becomes meaningful alongside the rest of your thyroid values.

This glossary entry is for informational purposes only and is not a substitute for professional medical advice. Read our Terms of Use.

References

Clinical "Triiodothyronine (T3) Tests" MedlinePlus (NIH/NLM). medlineplus.gov/lab-tests/triiodothyronine-t3-tests.

"Thyroid Function Tests" American Thyroid Association (ATA). www.thyroid.org/thyroid-function-tests.

About our sources

Official sources

Government health agencies, national institutes, and intergovernmental bodies whose guidelines carry regulatory or policy weight.

MedlinePlus (NIH / U.S. National Library of Medicine) NIH Office of Dietary Supplements World Health Organization Centers for Disease Control and Prevention NCBI Bookshelf / StatPearls (NIH/NLM) NHLBI, National Institutes of Health NIDDK, National Institutes of Health National Cancer Institute (NIH) National Health Service (UK) National Institute for Health and Care Excellence European Food Safety Authority

Clinical / professional

Medical societies, specialty colleges, and professional associations whose consensus statements reflect clinical practice standards.

Testing.com / Lab Tests Online (ADLM, ex-AACC) Mayo Clinic Laboratories Cleveland Clinic MSD Manuals American Diabetes Association American Heart Association Endocrine Society American Thyroid Association

Peer-reviewed research

Original research articles and systematic reviews published in indexed journals and verified through PubMed or DOI.

History of updates

Published · Jul 13, 2026

First published; written and independently fact-checked against the cited sources.

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