Free T3 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 T3:
- Optimal 3.2 – 4.2 pg/mL (4.9 – 6.5 pmol/L); sufficient 2.5 – 4.5 pg/mL (3.8 – 6.9 pmol/L); out of range below 2.5 or above 4.5 pg/mL.
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 |
|---|---|---|---|
| pg/mL | 3.2 – 4.2 | 2.5 – 4.5 | below 2.5 or above 4.5 |
| pmol/L | 4.9 – 6.5 | 3.8 – 6.9 | below 3.8 or above 6.9 |
What the evidence says
The acceptable band sits within published adult laboratory intervals (Labcorp 2.0 to 4.4 pg/mL; UK laboratories 3.7 or 3.8 to 6.0 or 6.6 pmol/L). Assay-dependent: intervals differ between platforms, and some laboratories set the lower limit at 2.0 pg/mL, 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.
Assay: Results differ meaningfully between platforms and methods. Compare against the interval printed by the issuing laboratory before acting on a borderline value.
Sources
- Triiodothyronine (T3), Free — Labcorp, 2026 · Laboratory / reference-interval sourceAdult interval of 2.0 to 4.4 pg/mL. Supports Sera's upper edge of 4.5 after rounding; the lower limit is 2.0 against Sera's 2.5.
- Free T3 — Worcestershire Acute Hospitals NHS Trust, 2026 · Laboratory / reference-interval sourceAdult interval of 3.8 to 6.0 pmol/L for men and women on a Beckman Coulter assay. The lower limit equals Sera's 3.8 pmol/L; the upper limit is below Sera's 6.9.
- Thyroid Function Tests — North Bristol NHS Trust, Severn Pathology, 2026 · Laboratory / reference-interval sourceFree T3 interval of 3.7 to 6.6 pmol/L, close to Sera's 3.8 to 6.9 pmol/L.
What Free T3 (FT3) measures
The most active thyroid hormone, which directly drives your metabolism.
The body has to convert T4 into T3, and this often does not work well. Low Free T3 with a normal TSH and Free T4 can explain ongoing fatigue, brain fog, and trouble with weight.