Definition
What Free T3 measures
Free T3 is the unbound, biologically active fraction of triiodothyronine. Most T3 is converted from T4 in tissues; the blood value is one window into that process.
Illustrative sample band · Synthetic
3.3pg/mL
1.8Sample target 3–4.2 pg/mL4.6
Why it matters
How clinicians usually read it
It helps when TSH and symptoms do not match. Isolated Free T3 changes are easy to over-read without Free T4 and the clinical story.
When it reads low
A lower value can mean
A lower Free T3 can appear in illness, calorie restriction, or reduced T4-to-T3 conversion. It is not, by itself, a treatment instruction.
When it reads high
A higher value can mean
A higher Free T3 can accompany over-replacement or hyperthyroid states. Assay differences make serial results from the same lab more useful.
Questions for a clinician
Bring the number into a real conversation
01
How should Free T3 be read with TSH and Free T4, not alone?
02
Could a recent illness or a low-calorie stretch explain this?
03
Is the same laboratory and assay being used for comparison?
Educational boundary
Educational information only. Displayed ranges are illustrative sample bands, not a diagnosis, treatment plan, or substitute for the contracted laboratory report and a qualified clinician.
See it as a sample result
The sample dashboard shows an illustrative reading, target band, and trend for Free T3.