Total T4 vs Free T4: what's the difference?
Educational — not medical advice · Ranges vary between laboratories
The short answer: Total T4 measures all thyroxine in the blood, most of it attached to carrier proteins. Free T4 measures only the small unbound fraction your tissues can actually use. That makes total T4 sensitive to anything that shifts protein levels, while free T4 tracks true thyroid activity more cleanly.
Total T4 measures all thyroxine circulating in the blood, most of it attached to carrier proteins, with only a small free fraction available to tissues. It gives a general sense of thyroid hormone output but is more easily influenced by non-thyroid factors than free T4.
Full Total T4 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
| Total T4 | Free T4 | |
|---|---|---|
| Panel | Thyroid panel | Thyroid panel |
| Unit | µg/dL | ng/dL |
| Typical adult range | Normal 5–12 µg/dL | Normal 0.8–1.8 ng/dL |
| Also known as | total thyroxine, TT4 | free thyroxine, FT4 |
General adult reference intervals — your lab's printed range takes precedence.
Why they're often tested together
Free T4 has largely become the standard, and total T4 turns up mainly on older or broader panels. When both appear, a total T4 outside range alongside a normal free T4 usually points to a binding-protein effect — pregnancy, estrogen-containing medicines, or a liver or kidney condition — rather than the thyroid itself.
Questions people ask
Does an abnormal total T4 always mean a thyroid problem?
Not necessarily. Because it counts bound hormone as well as free hormone, a value outside range can reflect a change in carrier protein levels from pregnancy, certain medicines, or a liver or kidney condition rather than any shift in thyroid function.
Why did free T4 become the preferred test?
It measures only the active, unbound hormone, so it isn't thrown off by protein-level changes that have nothing to do with the thyroid. That generally gives a more reliable read on thyroid status, which is especially useful during pregnancy.
More comparisons
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