Lab test glossary · Thyroid
Free T3: what it means
Also known as: FT3, free triiodothyronine
Free T3 is the most active thyroid hormone; it's sometimes added to TSH and free T4 to complete the thyroid picture.
What it measures
Free T3 (triiodothyronine) is the most metabolically active thyroid hormone available to your tissues. Most thyroid screening relies on TSH and free T4, but free T3 is sometimes added — for instance when hyperthyroidism is suspected — to round out the assessment.
Typical range
Often roughly 2.3–4.2 pg/mL, varying by lab and assay.
Reference ranges differ between labs and are printed on your own report — always read your result against that range, not a general one.
What a high result can mean
- Overactive thyroid
- Too much thyroid-hormone replacement
A high value is a starting point for a conversation, not a diagnosis — many causes are benign or temporary.
What a low result can mean
- Underactive thyroid
- Serious non-thyroid illness can lower it temporarily
Why the trend matters
Free T3 is interpreted alongside TSH and free T4; the pattern among the three, over time, is what guides decisions.
When to talk to your doctor
Interpret free T3 with your doctor and your other thyroid tests together, not on its own.
This page is general education about a common lab test — it isn’t medical advice and can’t interpret your specific result. What a value means depends on your history, symptoms, and the rest of your labs, which is a conversation for you and your clinician.