Free T3 vs Free T4: what's the difference?
Educational — not medical advice · Ranges vary between laboratories
The short answer: Free T4 is the thyroid's main output. Free T3 is the more biologically potent hormone, and much of it is made by converting T4 in the liver and other tissues. So free T4 reflects gland output, while free T3 reflects that output plus how well the conversion step is working.
Free T3 is the unbound, active form of triiodothyronine, the more potent of the two main thyroid hormones. Much of the body's T3 comes from converting T4 in the liver and other tissues, so it reflects both thyroid output and how well that conversion is working.
Full Free T3 guideFree T4 is the portion of thyroxine, one of the two main thyroid hormones, that circulates unattached to carrier proteins and is available for the body's cells to use. It reflects how much active thyroid hormone is currently on hand.
Full Free T4 guideAt a glance
| Free T3 | Free T4 | |
|---|---|---|
| Panel | Thyroid panel | Thyroid panel |
| Unit | pg/mL | ng/dL |
| Typical adult range | Normal 2.3–4.2 pg/mL | Normal 0.8–1.8 ng/dL |
| Also known as | free triiodothyronine, FT3 | free thyroxine, FT4 |
General adult reference intervals — your lab's printed range takes precedence.
Why they're often tested together
Free T4 is ordered far more routinely, usually added when TSH is abnormal. Free T3 is kept for specific situations, such as a suspected overactive thyroid or when the TSH and free T4 pattern doesn't explain someone's symptoms. All three are interpreted together rather than one at a time.
Questions people ask
Can free T3 be low with a healthy thyroid?
Yes. During significant illness or very low food intake, the body can deliberately slow T4-to-T3 conversion to conserve energy. This non-thyroidal illness pattern usually settles as the underlying illness improves, rather than reflecting a true thyroid problem.
Why isn't free T3 on every thyroid panel?
TSH and free T4 already capture the common patterns, and a free T3 result is hard to interpret in isolation. It tends to be reserved for the questions those first two tests leave open, such as a suspected overactive thyroid.
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