Free T4: what it means.
Circulating thyroid hormone.
Written to our editorial standards · reviewed against published research· Updated 2 Jun 2026
Typical reference bands (ng/dL)
Lowunder 0.8 ng/dL
Optimal1-1.6 ng/dL
Highover 1.8 ng/dL
Ranges vary by laboratory, age, and sex, your lab's own reference range always takes precedence.
Common causes
- A low free T4 points to an underactive thyroid
- A high free T4 points to an overactive thyroid or over-replacement
What it can feel like
- Low T4 tends toward sluggishness, weight gain, and cold intolerance
- High T4 tends toward agitation, palpitations, and heat intolerance
Who should test
Usually interpreted together with TSH. Any abnormality belongs in a clinician's hands.
See your own Free T4
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Related biomarkers
Common questions
What does a low Free T4 mean?
Common drivers include a low free T4 points to an underactive thyroid; a high free T4 points to an overactive thyroid or over-replacement. A result outside the optimal range is best read in context: discuss it with your clinician, who can weigh the full picture and your lab's own reference range.
How is Free T4 best supported?
Through diet, lifestyle, and addressing the underlying cause with your clinician. Upload your labs to suppdoc for a tailored read.
Educational use only, not medical advice or diagnosis. Always interpret lab results with a qualified clinician.