ThyroidReported in ng/dLFree thyroxine (Free T4)
The unbound fraction of T4, the hormone the thyroid mainly secretes. Free T4 is the share available to tissues.
What it measures
Most T4 in blood is bound to proteins and is not immediately available. Free T4 is the small unbound portion. It is the T4 measurement that best matches thyroid production, because it is less pushed around by changes in binding protein than a total T4 would be. Assays differ, so the interval printed on your result is the one to use.
Why it is on the panel
Free T4 explains a TSH that is outside its interval. High TSH plus low free T4 supports hypothyroidism. Low TSH plus high free T4 supports hyperthyroidism. TSH changes with a still-typical free T4 are the “subclinical” patterns, which are common and are interpreted in context.
How to read it
Start with TSH, then free T4, then total T3 if the picture is incomplete. Heart rate, weight trend, temperature tolerance, and cholesterol sometimes move with a real thyroid shift. A high cholesterol and a high TSH can be the same story.
When it runs higher
Hyperthyroidism, thyroiditis in its release phase, and taking too much thyroid hormone raise free T4. Biotin supplements can interfere with some thyroid assays and produce a confusing result. Mention supplements when you review the report.
When it runs lower
Hypothyroidism and some medicines, including lithium and amiodarone, can lower free T4. Acute illness can shift thyroid tests temporarily. A clinician decides whether a low result needs treatment or a repeat draw.
Same group
Other Thyroid tests
How to use this page
This page explains what the measurement is for. It is educational. Your Healiam report includes the reference interval for your age and sex, and a clinician reads that result with your history, medicines, and the rest of the panel. Talk with a clinician before changing diet, supplements, or treatment because of a number on a report.
Free T4
Free T4 is part of the full panel.
No referral or insurance required.
One preventive draw covers this measurement and the rest of the Healiam panel. CLIA-certified results come back in days.