TBG 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 TBG, which differ for men and women:
- Male: Optimal 12 – 26 μg/mL (12 – 26 mg/L); out of range below 12 or above 26 μg/mL.
- Female: Optimal 11 – 27 μg/mL (11 – 27 mg/L); out of range below 11 or above 27 μg/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 | Applies to | Optimal | Sufficient | Out of range |
|---|---|---|---|---|
| μg/mL | Male | 12 – 26 | — | below 12 or above 26 |
| μg/mL | Female | 11 – 27 | — | below 11 or above 27 |
| mg/L | Male | 12 – 26 | — | below 12 or above 26 |
| mg/L | Female | 11 – 27 | — | below 11 or above 27 |
No separate Sufficient band is currently defined for this marker.
What the evidence says
The bands are Mayo Clinic Laboratories' adult intervals: 12 to 26 μg/mL for men and 11 to 27 for women. μg/mL and mg/L are the same number. A Dutch hospital laboratory (analysed at Erasmus MC) agrees for men but gives women up to 38 mg/L, most likely because its population includes women on oestrogen, which raises TBG. TBG explains a total T4 or T3 that disagrees with the free hormone and is not a target in itself, so there is no optimal range. No NHS trust publishes an interval; UK samples are usually sent away.
Sources
- Thyroxine-Binding Globulin (TBG), Serum (TBGI) — Mayo Clinic Laboratories (performing laboratory), Duke University Hospital test catalog, 2026 · Laboratory / reference-interval sourceAdult intervals: men 12 to 26 μg/mL, women 11 to 27 μg/mL. The basis of Sera's bands.
- K-VZ TBG (Thyroxine-binding globulin) - Labgids — Ziekenhuis Slingeland (Netherlands), analysed at Erasmus MC, 2017 · Laboratory / reference-interval sourceEuropean intervals: men 12 to 23 mg/L, women 14 to 38 mg/L. Confirms the sex split and the male low edge; women's upper edge is much higher than Mayo's.
What Thyroxine-Binding Globulin (TBG) measures
The main protein that carries thyroid hormone in the blood.
Explains a total T4 or T3 that disagrees with the free hormone. Oestrogen, pregnancy and the contraceptive pill raise TBG, which raises total hormone while free hormone stays normal; androgens, steroids and liver or kidney disease lower it. It is not a measure of thyroid function in itself.