Free T4 optimal range and reference range
Compiled by Alina Radu, founder of Sera, from the published sources below · Last reviewed November 9, 2026
Sera's bands for Free T4:
- Optimal 1.1 – 1.5 ng/dL (14 – 19 pmol/L); sufficient 0.9 – 1.8 ng/dL (12 – 23 pmol/L); out of range below 0.9 or above 1.8 ng/dL.
Evidence grade: Published reference interval. A band that primarily reflects a published adult laboratory or population reference interval. It is not necessarily applicable to every assay or laboratory.
All units
| Unit | Optimal | Sufficient | Out of range |
|---|---|---|---|
| ng/dL | 1.1 – 1.5 | 0.9 – 1.8 | below 0.9 or above 1.8 |
| pmol/L | 14 – 19 | 12 – 23 | below 12 or above 23 |
What the evidence says
The acceptable band follows US laboratory reference intervals for adults (Mayo 0.9 to 1.7 ng/dL, Labcorp 0.82 to 1.77). Assay-dependent: intervals differ between platforms, and UK laboratories on Beckman assays report 8.0 to 18.0 pmol/L, so compare against the issuing laboratory's own interval before acting on a borderline value. The optimal band is a functional target with no published threshold behind it; pooled cohort data found the lowest cardiovascular and mortality risk in the low-normal part of the range, around 13.5 to 14.8 pmol/L, rather than the upper half.
Assay: Results differ meaningfully between platforms and methods. Compare against the interval printed by the issuing laboratory before acting on a borderline value.
Sources
- FRT4 - Overview: T4 (Thyroxine), Free, Serum — Mayo Clinic Laboratories, 2026 · Laboratory / reference-interval sourceAdult free T4 reference values of 0.9 to 1.7 ng/dL. Sera's lower edge of 0.9 matches; its upper edge is 1.8.
- Thyroxine (T4), Free, Direct — Labcorp, 2026 · Laboratory / reference-interval sourceAdult interval of 0.82 to 1.77 ng/dL. Supports Sera's upper edge of 1.8 after rounding; the lower limit is a little below Sera's 0.9.
- Free T4 (FT4) — Worcestershire Acute Hospitals NHS Trust, 2026 · Laboratory / reference-interval sourceAdult interval of 8.0 to 18.0 pmol/L on a Beckman Coulter assay. Both limits are well below Sera's 12 to 23 pmol/L, which is why the marker is assay-dependent.
- The optimal healthy ranges of thyroid function defined by the risk of cardiovascular disease and mortality: systematic review and individual participant data meta-analysis — Xu Y, Derakhshan A, Hysaj O, et al. — The Lancet Diabetes & Endocrinology, 2023 · Peer-reviewed publicationRisk of cardiovascular disease and death was lowest between the 20th and 40th percentiles of free T4, with medians of 13.5 and 14.8 pmol/L, and rose in the top fifth. That lowest-risk zone lies at the bottom of Sera's optimal band of 14 to 19 pmol/L, not across it.
What Free T4 (FT4) measures
The free, active form of thyroxine that is available for your cells to use.
Low Free T4 means the thyroid is not making enough hormone. Many people with thyroid conditions have low-normal Free T4 and still feel unwell.