Free T3
Also called Free triiodothyronine, FT3
Free T3 (free triiodothyronine) is the portion of the thyroid hormone T3 that circulates unattached to carrier proteins and can enter tissues, where it acts. T3 is one of the two hormones the thyroid makes, alongside thyroxine (T4); most T3 in the blood is bound to proteins, and only the free fraction is in the active form tissues can use.
T3 can be measured as free T3 or as total T3, which counts bound and free together. MedlinePlus notes that many experts consider total T3 the more accurate measurement, so laboratories and clinicians differ in which they order. Either way, T3 is rarely read alone: it is usually requested with TSH and T4, most often to investigate an overactive thyroid or to monitor thyroid treatment. What a low free T3 alongside a normal TSH can and cannot mean is covered in Free T3: What a Low Result Means When TSH Is Normal.
Why it matters after 45
Thyroid problems become more common with age, and the symptoms of too much or too little thyroid hormone (fatigue, poor sleep, weight change, a fast or irregular heartbeat, anxiety or low mood) are easy to mistake for midlife or menopause. Free T3 tends to come up when someone has those symptoms and a TSH result that does not seem to explain them, or when thyroid medicine is being adjusted.
Where it comes up
Related terms
References
- MedlinePlus. Triiodothyronine (T3) Tests. U.S. National Library of Medicine. https://medlineplus.gov/lab-tests/triiodothyronine-t3-tests/
- National Institute of Diabetes and Digestive and Kidney Diseases. Thyroid Tests. https://www.niddk.nih.gov/health-information/diagnostic-tests/thyroid
Definitions here are educational and are not medical advice. They explain what a term means; the pages linked above cover what it means for you, and a clinician reviews your own results. Editorial & medical review policy · All glossary terms