Total T4 blood test in Chiswick
A blood test measuring total T4 (thyroxine), the main hormone made by the thyroid, in both bound and free forms, for people with symptoms such as tiredness, weight change or feeling cold, or for thyroid monitoring.
What the test measures
Thyroxine (T4) is the main hormone released by the thyroid gland. It acts as a reservoir that the body converts into the more active T3 as needed. Together these hormones regulate metabolism, energy, body temperature, heart rate and many other processes.
Almost all T4 in the blood is attached to carrier proteins, mostly thyroxine-binding globulin, with only a tiny free fraction available to cells. Total T4 measures both. Because it rises and falls with carrier protein levels, the NHS now more commonly uses free T4, but total T4 remains useful in some situations, particularly when interpreted together with TSH and binding protein results.
The price includes the sample taken at our Chiswick clinic, analysis at the Randox accredited laboratory, a practitioner review of your result, and a GP feedback report on request.
Who it is for
- You have tiredness, weight gain, dry skin or feel cold and want a thyroid check.
- You have weight loss, a fast heartbeat or feel anxious and want to explore thyroid causes.
- You take levothyroxine and would like an additional measurement to discuss with your GP.
- Your free T4 result does not fit your symptoms and your practitioner would like a total level.
- You are reviewing several thyroid markers to build a fuller picture.
Understanding your result
Your report shows your total T4 level with the laboratory reference range. A low result may suggest an underactive thyroid, but it can also reflect lower carrier proteins, for example with some medicines or illness. A high result may suggest an overactive thyroid or may reflect higher carrier proteins, as in pregnancy or with oestrogen treatment.
Your practitioner will read total T4 alongside TSH and, where needed, free T4 and thyroxine-binding globulin. A value outside the range is something to explore with your practitioner, not a diagnosis in itself.
Preparing for your test
Eating beforehand will not affect this test, so there is no need to fast. If you take levothyroxine, have the test before your daily dose. Avoid biotin supplements for at least 48 hours beforehand, as biotin can interfere with thyroid immunoassays. Please tell us if you are pregnant or take the contraceptive pill, HRT, steroids or other medicines that affect the thyroid.
For a complete thyroid assessment, our thyroid blood tests page includes TSH, free T4, free T3 and antibody tests. A thyroxine-binding globulin test is also available separately. See every blood test and price in Chiswick.
Questions
How much is a total T4 test in Chiswick?
The Total T4 test costs £49. This includes the blood sample at our Chiswick clinic, analysis at the Randox accredited laboratory, a practitioner review of your result, and a GP feedback report on request.
Is total T4 the same as free T4?
No. Total T4 includes hormone bound to proteins, while free T4 measures only the active unbound portion. Free T4 is the test most commonly used in the NHS.
Can medicines change my total T4?
Yes. Oestrogen, the contraceptive pill and some other medicines change carrier proteins and therefore total T4. Let us know what you take.
How often should I check my thyroid if I take levothyroxine?
Your GP will usually advise a check at least once a year once your dose is stable, mainly using TSH.
