T3 Uptake 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 T3 Uptake:
- Optimal 24 – 39 %; out of range below 24 or above 39 %.
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 |
|---|---|---|---|
| % | 24 – 39 | — | below 24 or above 39 |
No separate Sufficient band is currently defined for this marker.
What the evidence says
T3 uptake is not a measure of T3. It estimates how many free binding sites the carrier proteins (mainly thyroxine-binding globulin) have, and its use is to correct total T4 for binding, which gives the free thyroxine index. The band is 24 to 39 %, Labcorp's adult interval, or 0.8 to 1.3 where the laboratory reports it as a binding ratio. A low uptake usually means more binding protein, as with oestrogen, the pill or pregnancy, and a high uptake means less, as with androgens, high-dose steroids or protein loss. It is not read alone. Free T4 measured directly has largely replaced it, and it is mainly seen on older thyroid panels. Biotin supplements can distort the result.
Sources
- T3 Uptake (test 001156) — Labcorp test directory, 2026 · Laboratory / reference-interval sourceAdult (over 18) interval 24 to 39 % for men and women. The test reflects available binding sites on thyroxine-binding globulin, is not a measurement of serum T3, and is used with total T4 to give the free T4 index.
- ABIM Laboratory Test Reference Ranges, January 2026 — American Board of Internal Medicine, 2026 · Laboratory / reference-interval sourceUS reference table: T3 resin uptake 25 to 35 %. Narrower than Labcorp's, cited as a comparison.
- Thyroxine (T4), Uptake, Serum (test TUP) — NorDx Laboratories (MaineHealth), 2026 · Laboratory / reference-interval sourceRoche electrochemiluminescence assay reported as a binding ratio: reference values 0.8 to 1.3 TBI. Biotin falsely raises the result.
- T Uptake (Thyroxine Binding Capacity) (test 918) — Northwell Health Laboratories, 2026 · Laboratory / reference-interval sourceSame assay family, reported both ways: ratio 0.8 to 1.3 TBI units and percent 28 to 41 %.
What T3 Uptake measures
An estimate of how many free binding sites thyroid-binding proteins have, used to correct total T4 for binding. Not a measure of T3.
Low when binding proteins are raised (pregnancy, oestrogen, the pill) and high when they are low, so it explains a total T4 that does not match free T4.