Thyroid . Patient guide
Free Thyroxine (Free T4) Blood Test
Also known as: fT4, Unbound T4
What is Free T4
Free T4 is the unbound, biologically active form of thyroxine, the main hormone produced by the thyroid gland. According to the Roche Elecsys assay used widely in UK labs, the typical adult reference range is approximately 12 to 22 pmol/L; per NICE CKS Hypothyroidism, a low free T4 with raised TSH supports a diagnosis of primary hypothyroidism, while a high free T4 with suppressed TSH supports primary hyperthyroidism. Free T4 is the partner marker to TSH for distinguishing primary thyroid disease from pituitary or assay-interference causes (high-dose biotin interferes with free T4 immunoassays; hold for 48 to 72 hours before testing).
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.
Looking to book a test?
Comprehensive private thyroid blood test
TSH, Free T4, Free T3 + TPO antibodies. NICE-aligned interpretation by GMC-registered doctors.
Symptoms often investigated with Free Thyroxine
Free Thyroxine is commonly tested when patients present with the following symptoms. If any of these resonate with you, the linked guides explain what to look for and which test pathway is appropriate.
Read this marker alongside another
Free Thyroxine is most useful when interpreted together with the markers below. Each guide walks through the 4-quadrant matrix our clinicians use when both come back at once.
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.
Common questions about Free T4
What is a normal Free T4 range?
Typical adult range: 12 to 22 (pmol/L (varies slightly between UK labs)). Always interpret your own results against the laboratory range printed on your report, since assay-specific reference ranges vary.
What does a low Free T4 result mean?
Hypothyroidism (often with raised TSH), severe illness (“non-thyroidal illness”), or pituitary disease.
What does a high Free T4 result mean?
Hyperthyroidism (Graves, toxic nodule, thyroiditis) or over-replacement on levothyroxine.
Do I need to fast or prepare for the Free T4 blood test?
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.
Can I order a Free T4 blood test privately in London?
Yes. WMG Health offers Free T4 as part of bespoke panels and several pre-built panels at our 134 Harley Street clinic. Results are clinician-reviewed by a GMC-registered doctor within 4 hours for the most common assays. All panels are custom-built around your specific question; bookings via /contact/ or 020 3239 3378.