What is Free T3?
your most potent thyroid hormone
Free T3 is the most active thyroid hormone. In hyperthyroidism T3 often rises before T4. In low T3 syndrome T3 may be low while TSH and T4 are normal.
T3 is formed mainly by conversion from T4 in body tissues. The test is useful when thyroid disease is suspected but T4 and TSH do not give a clear picture.
Test facts
- Unit
- pmol/L
- Sample
- Serum
- Fasting
- No fasting needed
- Turnaround
- 2 days
What does low Free T3 mean?
Low free T3 can appear in hypothyroidism or in severe illness where conversion from T4 is impaired.
What does high Free T3 mean?
High free T3 indicates hyperthyroidism.
Panels that measure Free T3
- Deep Baseline
- Hormone+
- Thyroid Complete
- Thyroid + Autoimmune
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.