Free T4, the storage hormone Free Thyroxine (FT4)
The main thyroid hormone floating free and ready to be used.
Also printed asFree Thyroxine · Free T4 · FT4
What it measures
The unbound, usable share of the storage form of thyroid hormone.
Read with TSH it tells you whether the thyroid gland itself, or the signal to it, is the problem.
Why it matters for how long you stay well
Read with TSH it separates a gland that is genuinely underperforming from a brain that is simply asking louder, and only the first of those changes your decades.
How it shows up in daily life
It tracks how much fuel your thyroid is actually delivering, which you feel as warmth, energy and how fast things move through you.
High and low readings
When it reads high
An overactive thyroid pattern.
When it reads low
An underactive thyroid pattern.
What moves it
Daily life
- iodine intake
- selenium intake
- serious illness, which lowers it
- pregnancy
Medicines known to shift it
- thyroid hormone replacement (raises it)
Listed so a result can be read in context. It is not advice to start, stop or change any medicine; that decision belongs with your physician.
Read it with
Markers that move this one, or that this one moves. One number read alone can mislead.
- LDL, the carrying cholesterolKnown mechanism
As this rises, LDL falls as it rises
Free T4 is the hormone that actually gives the order. More of it speeds the liver's clearing of LDL, and less of it lets LDL linger in the blood.
Common questions
What does Free Thyroxine (FT4) measure?
The unbound, usable share of the storage form of thyroid hormone. Read with TSH it tells you whether the thyroid gland itself, or the signal to it, is the problem.
What does a high Free T4 result usually mean?
An overactive thyroid pattern.
What does a low Free T4 result usually mean?
An underactive thyroid pattern.
What is the usual reference range for Free T4?
A broad adult reference band is 0.8–1.8 ng/dL. Labs differ, so read your result against the band printed on your own report.
Educational only, not a diagnosis. A single number means little without your history and the rest of your panel, so discuss your own results with your physician.
Typical adult reference band
- Adults
- 0.8–1.8 ng/dL
Labs differ. The band printed on your own report is the one that applies to your result.
Reported in ng/dL, pmol/L
Ageing relevance
2/5
Relevant to the ageing of one organ, or to disease screening.
Worth knowing
Most thyroid hormone travels stuck to protein; only the free part can actually do anything.
Have a report with this marker on it? Read it in context.
Upload it to Oraia, our health-intelligence tool, for a plain-language read of your markers that a Genoryx doctor can review.
Your biological age is a number you can measure.
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