Thyroid . Patient guide
Free Triiodothyronine (Free T3)
Also known as: fT3, Unbound T3
Free T3 is the unbound, biologically active form of triiodothyronine, the more potent of the two main thyroid hormones. Most T3 is produced from T4 in peripheral tissues. Free T3 sometimes flags thyroid problems before TSH and free T4 turn frankly abnormal.
This biomarker entry is being clinically reviewed by our team. The factual content draws on UK guidance (NICE, NHS, Royal Colleges and the relevant speciality society where cited).
Reference range
Reported in pmol/L. Final reports always carry the issuing laboratory's range, which is what your clinician will interpret against.
| Group | Range | Note |
|---|---|---|
| Typical adult range | 3.1 to 6.8 | pmol/L (varies slightly between UK labs) |
What it is
T3 is roughly four times more potent than T4 and is what most tissues respond to. The thyroid produces a small amount of T3 directly; most is generated by enzymatic conversion of T4 in the liver, kidneys and other tissues.
Why a clinician would order it
Used as part of a full thyroid screen, particularly where symptoms are present but TSH is borderline. Useful in evaluating T3 toxicosis and rare conversion problems where T4 is normal but T3 is low.
If your level is outside the range
Symptoms of low Free T3
- Fatigue
- Cold intolerance
- Mental fog
- Low mood
- Constipation
What low can indicate. Hypothyroidism, non-thyroidal illness, severe calorie restriction.
Symptoms of high Free T3
- Anxiety, palpitations
- Weight loss
- Heat intolerance
- Tremor
What high can indicate. Hyperthyroidism (including T3 toxicosis), excess T3 supplementation.
Testing tips
No fasting required. Biotin at high doses can interfere with the assay; stop biotin for 48 hours before testing.
Where you can get this tested
Free Triiodothyronine is included in the following WMG Health panels. Same-day appointments at our Harley Street clinic, with results clinician-reviewed.
Want a specific combination of markers we do not have a panel for? Build a custom panel and our clinicians will design one for you.
Sources
UK guidance our clinicians use when interpreting this marker.
This page is general patient information, not personal medical advice. A GMC-registered clinician will review your results and tailor any interpretation to you. See our Editorial Policy for how we write and review content.