Thyroid
Free T4
The unbound, active fraction of the main thyroid hormone.
Unit ng/dLTypical adult range 0.9–1.7 ng/dL
What it is
The unbound, active fraction of the main thyroid hormone.
Why it matters
It tells you whether a TSH that is out of range has started to affect hormone levels yet.
Reading the number
Read it with TSH, never alone. High-dose biotin supplements can distort the assay in either direction.
What moves it
biotin supplementsacute illnesspregnancythyroid medication timing
When to raise it
Worth discussing together with TSH rather than on its own.
Also known as
On a report in another language.
A lab report names this analyte in its own language. Vitals Vault recognises each of these and files them all under Free T4.
T4 LivreFT4Vaba T4Freies T4Tiroxina LivreT4L
Catalog entry last updated 2026-08-27. Browse all 165 metrics.