What it measures
All the triiodothyronine (T3) in the blood, both the part bound to carrier proteins and the small free part. T3 is the active thyroid hormone, made mostly from T4 in the body's tissues.
Why people test it
- Part of some full thyroid panels, alongside TSH and free T4.
- Measured when an overactive thyroid is suspected, as T3 can rise before T4 does.
Typical range
Ranges vary by lab, method, age and sex, and your report shows the lab's own range.
Finger-prick or vein?
Offered as a vein draw only, as part of larger panels that need more blood than a finger-prick gives.
Carrier protein levels change in pregnancy and with oestrogen in the contraceptive pill or HRT, which moves total T3 without changing the free hormone.