Total thyroxine (T4)
All thyroxine in the blood, bound and free together.
What it is
Total T4 counts both protein-bound and free hormone, so anything changing binding proteins - oestrogen, pregnancy, some medicines - moves it without thyroid function changing. Free T4 avoids that problem, which is why it is usually preferred.
Why it matters
Total T4 is interpreted with TSH and free hormone measurements, with binding-protein effects in mind.
This guide is educational only and cannot diagnose you. Discuss results, especially anything outside your laboratory’s reference interval, or that does not match how you feel, with a GP or other appropriately qualified clinician.
This guide is educational only and cannot diagnose you. Discuss results, especially anything outside your laboratory’s reference interval, or that does not match how you feel, with a GP or other appropriately qualified clinician.
How it affects the body
Thyroxine sets the pace of metabolism throughout the body.
For athletes
Androgens lower binding proteins and can lower total T4 while free levels stay normal - a known trap when reading TRT bloods.
Related tests
No published panels currently list this marker.