Total 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 Total T4:
- Optimal 6 – 10 μg/dL (77 – 129 nmol/L); sufficient 4.5 – 12 μg/dL (58 – 154 nmol/L); out of range below 4.5 or above 12 μg/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 |
|---|---|---|---|
| μg/dL | 6 – 10 | 4.5 – 12 | below 4.5 or above 12 |
| nmol/L | 77 – 129 | 58 – 154 | below 58 or above 154 |
| ng/dL | 6,000 – 10,000 | 4,500 – 12,000 | below 4,500 or above 12,000 |
What the evidence says
The acceptable band of 4.5 to 12 µg/dL is the adult laboratory reference interval for total T4 (Labcorp 4.5 to 12.0; Mayo Clinic Laboratories 4.5 to 11.7). The optimal band of 6 to 10 µg/dL is a functional target with no published study or guideline behind it. Total T4 follows binding proteins: pregnancy and oestrogen raise it without any change in thyroid function, so read it alongside TSH and free T4. Intervals vary by assay; the laboratory's own interval takes precedence.
Assay: Results differ meaningfully between platforms and methods. Compare against the interval printed by the issuing laboratory before acting on a borderline value.
Sources
- Thyroxine (T4) — Labcorp, 2026 · Laboratory / reference-interval sourceReference interval 4.5 to 12.0 µg/dL by electrochemiluminescence immunoassay. This is Sera's sufficient band exactly.
- T4 (Thyroxine), Total Only, Serum — Mayo Clinic Laboratories, 2026 · Laboratory / reference-interval sourceAdult reference values (20 years and older) 4.5 to 11.7 µg/dL by electrochemiluminescence immunoassay. Matches Sera's lower edge of 4.5 exactly; the upper edge is 0.3 µg/dL below Sera's 12.
What Total T4 measures
The total amount of T4 in your blood, both the bound and the free forms.
Helpful background when Free T4 is borderline. It can be pushed up or down by estrogen levels and by the proteins that carry thyroid hormone.