Free T4

Measured in ng/dL

The portion of the main thyroid hormone circulating unbound and available to your tissues — a snapshot that's most meaningful read next to TSH.

What it is

T4 (thyroxine) is the main hormone your thyroid releases, but most of it travels stuck to carrier proteins in the blood; 'free' T4 is the small unbound fraction that tissues can actually use. Measuring the free portion is meant to sidestep changes in those carrier proteins, which can shift with pregnancy, estrogen, and some illnesses — though the estimate isn't perfect and results vary between labs and assay methods. It's almost always interpreted alongside TSH rather than on its own, since the pair together tells a clinician more than either number alone.

Typical range

  • Adults0.8–1.8 ng/dL

Source · Reference-lab convention (LabCorp) (2025)

Reference ranges vary between laboratories, and what is normal for you depends on your own history. Your own lab’s range is the one printed on your report.

Worth asking your clinician

  • How does my Free T4 fit together with my TSH result?
  • Could anything I'm taking or anything going on with my health right now be affecting this measurement?
  • Is this a value worth rechecking over time, or is one reading enough in my situation?