Thyroid . Patient guide
Total T4 Blood Test
What is Total T4
Total T4 measures all of the thyroid hormone thyroxine in your blood, both the small active free fraction and the much larger portion bound to carrier proteins. It reflects thyroid output but is influenced by anything that changes those carrier proteins.
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 nmol/L. Final reports always carry the issuing laboratory's range, which is what your clinician will interpret against.
| Group | Range | Note |
|---|---|---|
| Adults | approx. 59 to 154 | nmol/L; varies between labs |
What it is
The thyroid makes thyroxine (T4), most of which travels bound to proteins, with only the free fraction biologically active. Total T4 sums both. Because pregnancy, the oestrogen in some contraceptives and other factors raise the binding proteins, free T4 has largely replaced total T4 for everyday thyroid assessment.
Why a clinician would order it
Total T4 is used as part of a thyroid assessment, particularly where a lab uses it in place of, or alongside, free T4, and in specific situations where binding-protein changes are relevant. It is always interpreted alongside TSH.
If your level is outside the range
Symptoms of low Total T4
- Tiredness
- Feeling cold
- Weight gain
- Low mood
- Dry skin (symptoms of an underactive thyroid)
What low can indicate. An underactive thyroid (hypothyroidism), read together with a raised TSH.
Symptoms of high Total T4
- Weight loss
- Palpitations
- Anxiety
- Heat intolerance (symptoms of an overactive thyroid)
What high can indicate. An overactive thyroid (hyperthyroidism), read together with a low TSH; a high total T4 with normal free T4 can simply reflect raised binding proteins (for example in pregnancy or on oestrogen).
Testing tips
No fasting needed. Interpret alongside TSH and, where available, free T4. Biotin supplements can interfere with some thyroid assays, so hold high-dose biotin for 48 hours before testing.
Looking to book a test?
Comprehensive private thyroid blood test
TSH, Free T4, Free T3 + TPO antibodies. NICE-aligned interpretation by GMC-registered doctors.
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 Total T4
What is a normal Total T4 range?
Adults: approx. 59 to 154 (nmol/L; varies between labs). Always interpret your own results against the laboratory range printed on your report, since assay-specific reference ranges vary.
What does a low Total T4 result mean?
An underactive thyroid (hypothyroidism), read together with a raised TSH.
What does a high Total T4 result mean?
An overactive thyroid (hyperthyroidism), read together with a low TSH; a high total T4 with normal free T4 can simply reflect raised binding proteins (for example in pregnancy or on oestrogen).
Do I need to fast or prepare for the Total T4 blood test?
No fasting needed. Interpret alongside TSH and, where available, free T4. Biotin supplements can interfere with some thyroid assays, so hold high-dose biotin for 48 hours before testing.
Can I order a Total T4 blood test privately in London?
Yes. WMG Health offers Total 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.