Free T3, the active hormone Free Triiodothyronine (FT3)
The active thyroid hormone, the one your cells actually use.
Also printed asFree Triiodothyronine · Free T3 · FT3
What it measures
The unbound, active thyroid hormone your cells actually use.
Drives energy, body temperature and heart rate directly; it falls first when you are ill, dieting hard or under strain.
Why it matters for how long you stay well
It drops during any serious illness or a very low-calorie phase even when the thyroid is healthy, so read it in the context of how the rest of your life has been going.
How it shows up in daily life
It is the hormone behind how warm you feel, how fast your heart runs and how quickly food moves through you.
High and low readings
When it reads high
An overactive thyroid pattern.
When it reads low
Poor conversion from T4; heavy dieting, illness and chronic stress all do this.
What moves it
Daily life
- very low-calorie dieting, which lowers it
- serious illness
- selenium intake
- liver health
Medicines known to shift it
- thyroid hormone replacement (raises it)
- steroids (lower 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.
Common questions
What does Free Triiodothyronine (FT3) measure?
The unbound, active thyroid hormone your cells actually use. Drives energy, body temperature and heart rate directly; it falls first when you are ill, dieting hard or under strain.
What does a high Free T3 result usually mean?
An overactive thyroid pattern.
What does a low Free T3 result usually mean?
Poor conversion from T4; heavy dieting, illness and chronic stress all do this.
What is the usual reference range for Free T3?
A broad adult reference band is 2.3–4.2 pg/mL. 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
- 2.3–4.2 pg/mL
Labs differ. The band printed on your own report is the one that applies to your result.
Reported in pg/mL, pmol/L
Ageing relevance
3/5
A moderate ageing association; used in clinical longevity panels.
Worth knowing
Most of it is made outside the thyroid, when other organs convert the storage hormone into the active one.
Have a report with this marker on it? Read it in context.
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