What it measures
Free T3 is the unbound part of triiodothyronine, the most active thyroid hormone. Most T3 is made in the body's tissues from T4. Free T3 is usually measured with TSH and free T4 rather than on its own.
Why people test it
- Checked when TSH is low, to look for an overactive thyroid.
- Used with TSH and free T4 when symptoms and other thyroid results do not match.
- Monitored in people on thyroid medicines, including liothyronine (T3).
- Can fall with very low energy intake, such as long cutting phases or under-fuelling in endurance sport.
Typical range
Ranges vary between labs with the method used, age and sex, and your report shows the lab's own range. One NHS trust's lab uses 3.5 to 6.8 pmol/L for adults. Source: Leeds Teaching Hospitals NHS Trust: Blood Sciences Reference Range Database v3.20.
Finger-prick or vein?
A finger-prick sample is usually fine for free T3.