Free T3
FT3
- Sample
- Blood
- Usual units
- pg/mL
Also known as FT3 · Free Triiodothyronine · FreeT3
What this test measures
Free T3 is the unbound portion of triiodothyronine, the thyroid hormone that acts most directly inside cells. A blood test measures this fraction because it is the hormone available to tissues, distinct from Total T3, which counts bound and unbound hormone together. It is usually reported in picograms per milliliter (pg/mL).
Reading your report
There isn’t one standard range for this test — each lab prints its own. Use the range shown next to your result.
Free T3 unit converter
Convert Free T3 units
Labs print this test in pg/mL or pmol/L depending on where the sample was run.
Related guides
- Guide
Why the same blood test has different "normal" ranges at different labs
Reference intervals are built from each lab's own population and equipment, which is why two labs can print different ranges for the same sample.
- Guide
Reference interval, cutoff, and the "optimal" numbers online: three different things
The range beside your result, the target a website quotes, and the threshold that defines a condition are built in different ways and answer different questions.
- Guide
Why the same lab printed a different reference range for you this year
Ranges are not fixed constants. The lab can change method, revise its reference group, or apply a range that depends on your age — none of which is an error.
- Guide
TSH, free T4 and free T3: why the first number isn't a thyroid hormone
TSH is made by the pituitary, not the thyroid, and moves opposite to the hormones it controls. What each row measures, and why free and total are different tests.