Insight into your Free T4 level.
Free T4 is the thyroid hormone thyroxine that circulates in your blood unbound to protein, and is therefore available to the tissues. A test gives insight into what your thyroid itself produces.
Also known as: FT4, free thyroxine
Interpreted by BIG-registered doctors
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Background
What is Free T4?
Your thyroid mainly produces thyroxine, or T4 for short. Most of it travels through the blood bound to transport proteins. Only the small portion that is unbound, the free T4, can enter the cells and do its work there. That is why laboratories measure Free T4 rather than total T4: the free fraction says the most about what is actually available to your tissues.
Free T4 is almost always assessed together with TSH. TSH is the signalling hormone from the pituitary gland, Free T4 is the product of the thyroid itself. The two respond to each other, and it is the combination that gives direction. Free T4 on its own is therefore mainly useful as an addition, or if you want to supplement an earlier TSH result. If you want a clear picture of thyroid function in one go, our Thyroid test (TSH + Free T4) is the usual starting point.
In brief
Free fraction
The portion not bound to protein
From the thyroid
The hormone the gland itself produces
Alongside TSH
The combination gives the most direction
Source of T3
Converted in the tissues into the more active T3
Common reasons to test
When can insight into your Free T4 be valuable?
Below are some situations in which measuring Free T4 can provide insight.
Insight into your thyroid hormone
A test gives a snapshot of the freely available thyroid hormone in your blood.
As an addition to a TSH result
If your TSH has already been measured and you want to know what the thyroid itself produces, Free T4 completes that picture.
With questions about energy or weight
Unexplained changes in energy, weight or sensitivity to temperature are sometimes associated with the thyroid.
For follow-up of thyroid medication
Those taking thyroid medication often have Free T4 measured alongside TSH to see whether their treatment remains stable.
With thyroid conditions in the family
Thyroid conditions run in families. With a family history, people more often choose periodic testing.
Before and during a wish to conceive
Thyroid function plays a role in fertility and pregnancy. A test may form part of preconception checks.
What your result means
How do we read your Free T4 level?
Free T4 is measured in picomoles per litre (pmol/L). The laboratory states the reference values that apply there alongside your result. Important: Free T4 is almost always assessed together with TSH. A Free T4 value on its own gives a limited picture and is not a diagnosis.
A low value
A low Free T4 can fit with an underactive thyroid, particularly in combination with an elevated TSH. Other causes sometimes play a role as well, for example serious illness, certain medication, or a problem with the signalling from the pituitary gland. Our doctor provides explanation and context with your result, so you can discuss it with your GP if you wish.
A normal value
A Free T4 within the laboratory's reference values gives a reassuring picture for this marker. Bear in mind that a normal Free T4 does not fully rule out a thyroid problem: in an early stage, TSH may already change while Free T4 is still normal. This is why the combination with TSH is customary.
An elevated value
An elevated Free T4 can fit with an overactive thyroid, often in combination with a reduced TSH. Thyroid medication at too high a dose, or temporary inflammation of the thyroid, can also raise the value. A single measurement is not a diagnosis. Our doctor explains what your value means and whether follow-up is customary.
How we interpret your result
Our doctors interpret your Free T4 against the laboratory's reference values and your personal context, such as symptoms, medication use, earlier results and any known conditions.
Related tests
Often combined with Free T4.
A single marker gives one perspective. For a broader picture you can combine Free T4 with a theme package or related markers.
Frequently asked questions
Everything you want to know
Do I need to fast for a Free T4 test?
No, you do not need to fast for Free T4. You can have blood drawn at any time of day.
What is the difference between TSH and Free T4?
TSH is the signalling hormone from the pituitary gland in the brain. Free T4 is the hormone the thyroid itself produces, in the form that is not bound to protein. TSH shows how strongly the body is stimulating the thyroid, Free T4 shows what the gland delivers. The two are almost always assessed together.
Why do you measure Free T4 and not total T4?
Most T4 in the blood is bound to transport proteins and is therefore not available to the cells. The amount of transport protein can also fluctuate, for example during pregnancy or with use of the contraceptive pill. Free T4 is less sensitive to those fluctuations and says more about what is available to the tissues.
Is Free T4 alone enough to assess my thyroid?
Usually not. A Free T4 value on its own gives a limited picture, because its meaning depends strongly on the accompanying TSH. If you want a clear picture of thyroid function in one go, choose the Thyroid test, in which TSH and Free T4 are measured together.
What is the difference between Free T4 and Free T3?
Free T4 is the hormone the thyroid mainly produces. Free T3 is the more active form, largely created from T4 in the tissues. Free T3 is measured as an addition for specific questions and can shift during illness or fasting, even without thyroid disease.
Does my medication affect the result?
Yes, some substances can affect the value or the measurement, including thyroid medication itself, biotin supplements, lithium, amiodarone and corticosteroids. Please tell us about your medication and supplements so our doctor can take this into account during interpretation.
How soon after a dose change can I retest?
After a change in thyroid medication, retesting usually takes place only after around 6 to 8 weeks. Retesting sooner is of little value, because the system needs time to stabilise.
What do I do with my result?
You receive a PDF report with your value and a personal explanation from a doctor. What you do with it, such as discussing it with your GP, remains entirely your choice.
Ready for insight into your Free T4 level?
For €59 you receive your Free T4 level including personal explanation from a doctor, typically within a few working days. No referral required.
Order Free T4 for €69 →Want to test multiple markers at once?
Combine Free T4 with other markers via Bloodworks Select. Blood draw costs are waived from €225.
69,-
All-in, blood draw included