Definition
What Free T4 measures
Free T4 is the unbound thyroxine available in blood. The thyroid releases mostly T4; tissues convert it to T3 as needed.
Illustrative sample band · Synthetic
1.2ng/dL
0.7Sample target 1–1.5 ng/dL1.9
Why it matters
How clinicians usually read it
Together with TSH it shows whether the control loop and the hormone supply agree. It is the usual confirmatory thyroid blood test after TSH.
When it reads low
A lower value can mean
Low Free T4 with a high TSH is the classic under-active pattern. Low Free T4 with a low TSH points to a different, pituitary-level question.
When it reads high
A higher value can mean
High Free T4 can reflect over-production or over-replacement. Biotin interference is a known lab confounder.
Questions for a clinician
Bring the number into a real conversation
01
Does this Free T4 agree with my TSH, or is the loop discordant?
02
Was I taking biotin or thyroid medication before the draw?
03
Do we need antibodies or a repeat before changing anything?
Educational boundary
Educational information only. Displayed ranges are illustrative sample bands, not a diagnosis, treatment plan, or substitute for the contracted laboratory report and a qualified clinician.
See it as a sample result
The sample dashboard shows an illustrative reading, target band, and trend for Free T4.