Free T4
What it measures, and what changes the number.
Thyroxine (T4) is the main hormone released by the thyroid gland. The body converts it into the more active T3. Most T4 travels bound to carrier proteins. The free T4 test measures the unbound portion.
Free T4 is typically ordered with TSH, and the two are interpreted as a pair. Free T3, total T4 and thyroid antibodies such as anti-TPO and anti-thyroglobulin are often added.
Free T4 is usually measured by immunoassay, and results vary between manufacturers. Reference methods such as equilibrium dialysis exist but are used less often. Biotin supplements, heparin, and changes in binding proteins (for example during pregnancy) can affect some assays. Reports may print it as T4, FREE or Tiroxina Libre (T4L), in pmol/L or ng/dL.
Reference ranges labs publish
Each lab states an interval for its own method and the population it measured. They differ, they are not interchangeable, and none of them is a verdict on a single result.
- Everyone, 20 years and over: 0.9 to 1.7 ng/dL (11.6 to 21.9 pmol/L) · ECLIA (immunoassay) · Source: Mayo Clinic Laboratories, FRT4 (T4 (Thyroxine), Free, Serum) (read 2026-09-24)
- Everyone, 20 years and over: 0.82 to 1.77 ng/dL (10.6 to 22.8 pmol/L) · ECLIA (immunoassay) · Source: Labcorp, 001974 (Thyroxine (T4), Free, Direct) (read 2026-09-24)
Usually reported in pmol/L or ng/dL.
Often read alongside: TSH, Free T3, Total T4, Anti-TPO antibodies, Anti-thyroglobulin antibodies
Calculators that use it: Free T3 to free T4 ratio