fT3 (free triiodothyronine)
fT3 (free triiodothyronine) is the most active thyroid hormone — produced mostly from fT4 in the tissues. It directly affects metabolism, heart rate and energy.
| Rating | Range | What it means |
|---|---|---|
| Talk to a doctor | below 3.1 pmol/L | Low fT3 — seek advice (thyroid). |
| Good | 3.1 – 6.8 pmol/L | Within the reference range. |
| Talk to a doctor | 6.8 pmol/L and above | Elevated fT3 — seek advice (thyroid). |
This marker also has 3 age- or sex-specific range sets in the app — the table shows the general one.
Why it matters
ETA: helps characterize the type of thyroid disorder.
If it’s high
High fT3 confirms an excess of thyroid hormones (hyperthyroidism) — accelerated metabolism, palpitations, tremor, weight loss.
What to do
Interpret fT3 with TSH and fT4. On its own it is not a lifestyle marker; a doctor interprets it in context.
Read together with
TSH · fT4 (free thyroxine) · Anti-TPO antibodies
Part of the guide: Thyroid →
Articles about this marker
- Thyroid: which tests tell you what, and how to do them properlyMarker explained
This content is for information only and is not a substitute for medical advice. Talk to your doctor about your results.
Your number, not the average
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