Thyroid
Free T4 (Free Thyroxine) — what it measures
For understanding, not diagnosis. Reference ranges differ between laboratories; the ranges on your own report, and your doctor’s reading of them, always come first.
What it measures
Free T4 measures the unbound, active fraction of thyroxine, one of the main hormones the thyroid gland produces. It is often measured alongside TSH to give a fuller picture of thyroid function.
What can skew it
Severe illness, pregnancy, and medicines or supplements such as high-dose biotin can distort free T4 measurements.
Reference ranges
Reference ranges vary by laboratory and by the assay method used, so read your value against your own report's range and review it with your clinician.
Medical disclaimer: This page is educational only. It cannot tell you whether your result is normal or what to do about it — that is a decision for your doctor, with your full history.