What is Free T4?
your main active thyroid hormone
Free T4 is the free, biologically active form of thyroxine. It complements TSH and is needed for a full picture of thyroid function.
T4 is the main hormone the thyroid produces. Most of it in the blood is protein-bound; only the free fraction is biologically active. When TSH is abnormal, free T4 helps determine whether disease is present and how severe it is.
Test facts
- Unit
- pmol/L
- Sample
- Serum
- Fasting
- No fasting needed
- Turnaround
- 2 days
What does low Free T4 mean?
Low free T4 indicates underactive thyroid.
What does high Free T4 mean?
High free T4 indicates overactive thyroid.
Panels that measure Free T4
- Deep Baseline
- Female Annual
- Male Annual
- Hormone+
- Perimenopause
- Pre-Conception
- Thyroid Basic
- Thyroid Complete
- Thyroid + Autoimmune
- Stress & Cortisol Basic
- Stress & Cortisol Full
More markers in the same area
Launching in Stockholm in 2026.
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This is general information about the marker, not an assessment of your values and not medical advice. Last updated: May 2026.