Free T3 (free triiodothyronine)
The most potent, unbound thyroid hormone - active in cells; sometimes tested when T4 results are unclear.
T3 (triiodothyronine) is the more biologically active of the two main thyroid hormones. Most T3 in circulation is made by the body converting T4 to T3 in peripheral tissues such as the liver and kidneys, rather than being secreted directly by the thyroid.
Free T3 (fT3) is the unbound, active form of T3. Like free T4, it is more clinically useful than total T3, which includes the large protein-bound fraction.
Free T3 is not always measured routinely. It tends to be tested when hyperthyroidism is suspected but free T4 is normal (a condition called T3 toxicosis, sometimes seen in early Graves' disease or toxic nodules), during treatment monitoring for hyperthyroidism, and in some specialist assessments.
Some people and clinicians advocate for wider free T3 measurement, particularly in hypothyroidism, arguing that T4-to-T3 conversion can vary between individuals. This remains a debated area; discuss with your clinician if you have concerns about your results and how you feel.
Related terms
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