Thyroid . Patient guide
Free Thyroxine (Free T4)
Also known as: fT4, Unbound T4
Free T4 is the unbound, biologically active form of thyroxine, the main hormone produced by the thyroid gland. It is measured alongside TSH to distinguish primary thyroid disease from pituitary or assay-interference causes.
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 | 12 to 22 | pmol/L (varies slightly between UK labs) |
What it is
Most circulating T4 is bound to plasma proteins and is unavailable to tissues. Only the small free (unbound) fraction enters cells and exerts hormonal effect. Measuring free T4 gives a more accurate picture than total T4.
Why a clinician would order it
Used in combination with TSH whenever symptoms suggest thyroid disease, when TSH is borderline, in the work-up of unexplained fatigue or weight change, and in monitoring patients on levothyroxine.
If your level is outside the range
Symptoms of low Free T4
- Fatigue
- Weight gain
- Cold intolerance
- Hair shedding
- Constipation
- Dry skin
What low can indicate. Hypothyroidism (often with raised TSH), severe illness (“non-thyroidal illness”), or pituitary disease.
Symptoms of high Free T4
- Anxiety, palpitations
- Weight loss despite normal appetite
- Heat intolerance
- Loose stools
- Tremor
What high can indicate. Hyperthyroidism (Graves, toxic nodule, thyroiditis) or over-replacement on levothyroxine.
Testing tips
No fasting required. Take levothyroxine after the blood draw, not before. Biotin supplements at high doses can interfere with the assay; stop biotin for 48 hours before testing.
Where you can get this tested
Free Thyroxine 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.