Insight into your Free T3 level.
Free T3 is the most active thyroid hormone, largely formed from T4 in the tissues. A test adds to TSH and Free T4 and gives insight into the hormone that does the work inside the cells.
Also known as: FT3, free triiodothyronine
Personal assessment by our BIG-registered Bloodworks® doctors
By the Bloodworks editorial team
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Background
What is Free T3?
T3, or triiodothyronine in full, is the thyroid hormone with the greatest effect inside the cells. Your thyroid produces only a small share of the T3 itself. Most of it is created elsewhere in the body, mainly in the liver, kidneys and muscles, by conversion of T4. As with T4, only the free fraction, unbound to protein, is available to the cells. That is why the laboratory measures Free T3.
Free T3 is an additional marker, not a starting point. Thyroid function is first assessed with TSH and Free T4. Free T3 comes into play for specific questions, for example when an overactive thyroid is suspected or when a result is difficult to place. One thing is important to know: Free T3 can fall during illness, fasting, a strict diet or considerable stress, without any thyroid disease being present. The body then temporarily slows the conversion of T4 into T3. An abnormal Free T3 therefore always calls for context.
In brief
Most active form
The hormone that does the work inside the cells
Formed from T4
Largely converted in liver, kidneys and muscles
Additional marker
Assessment starts with TSH and Free T4
Sensitive to context
Shifts with illness, fasting and stress
This marker is also included in
Testing several values in a single draw takes one blood collection and gives a more complete picture. The choice is yours.
Common reasons to test
When can insight into your Free T3 be valuable?
Below are some situations in which measuring Free T3 can provide insight.
Insight into the active hormone
A test gives a snapshot of the freely available, active thyroid hormone in your blood.
As an addition to TSH and Free T4
When a result raises questions, Free T3 offers an extra perspective on thyroid function.
When an overactive thyroid is suspected
With symptoms such as palpitations, restlessness, heat intolerance or weight loss, Free T3 is often added to the picture.
With questions about energy and metabolism
Persistent fatigue or changes in weight and sensitivity to temperature are sometimes associated with the thyroid.
For follow-up of thyroid medication
Those taking thyroid medication sometimes have Free T3 measured alongside TSH and Free T4 to complete the picture.
With thyroid conditions in the family
Thyroid conditions run in families. With a family history, people more often choose a broader thyroid picture.
What your result means
Free T3 too high or too low: what does your result mean?
Free T3 is measured in picomoles per litre (pmol/L). The laboratory states the reference values that apply there alongside your result. Important: Free T3 is an additional marker. The value can only be interpreted properly together with TSH and Free T4 and with your personal situation.
A low value
A low Free T3 can fit with an underactive thyroid, but also occurs without thyroid disease. During illness, recovery from surgery, fasting, a strict diet or considerable stress, the body temporarily slows the conversion of T4 into T3. That explains a share of the low results. Our doctor looks at your TSH, Free T4 and your context before drawing any conclusion.
A normal value
A Free T3 within the laboratory's reference values gives a reassuring picture for this marker. A normal Free T3 does not rule out a thyroid problem: with an early underactive thyroid, T3 often stays normal for a long time while TSH is already changing. This is why the combination with TSH and Free T4 remains leading.
An elevated value
An elevated Free T3 can fit with an overactive thyroid, often together with a reduced TSH. Thyroid medication or preparations containing thyroid hormone can also raise the value, as can biotin supplements, which may interfere with the measurement. 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 T3 against the laboratory's reference values and your personal context, such as symptoms, medication use, recent illness or dieting, and earlier thyroid results.
Related tests
Often combined with Free T3.
A single marker gives one perspective. For a broader picture you can combine Free T3 with a theme package or related markers.
+ €29 blood draw fee per orderFree from €225
Frequently asked questions
Everything you want to know
Do I need to fast for a Free T3 test?
No, you do not need to fast for Free T3. It is worth knowing, though, that prolonged fasting or a strict diet can temporarily lower the value.
What is the difference between Free T3 and Free T4?
Free T4 is the hormone the thyroid mainly produces itself. Free T3 is the more active form, largely formed from T4 in the tissues. T4 is therefore mainly the reserve, and T3 is the hormone that does the work inside the cells.
Is Free T3 alone enough to assess my thyroid?
No. Free T3 is an additional marker. Thyroid function is first assessed with TSH and Free T4. Without those two, a Free T3 value on its own is difficult to interpret.
Can my Free T3 be low without thyroid disease?
Yes. During illness, recovery from surgery, fasting, a strict diet or considerable stress, the body temporarily slows the conversion of T4 into T3. Free T3 can then come out lower without the thyroid itself having a problem. That is one of the reasons context matters so much with this marker.
When is it worthwhile to measure Free T3?
Mainly when an overactive thyroid is suspected, when symptoms do not fit the measured TSH and Free T4, or as an addition to an earlier thyroid test. For a first orientation, TSH with Free T4 is usually sufficient.
Does my medication affect the result?
Yes, some substances can affect the value or the measurement, including thyroid medication, biotin supplements, corticosteroids, amiodarone and certain beta blockers. Please tell us about your medication and supplements so our doctor can take this into account during interpretation.
How often should I have Free T3 measured?
A single measurement is enough for initial insight. After a change in thyroid medication, retesting usually takes place only after around 6 to 8 weeks, 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.
What is T3?
T3, or triiodothyronine in full, is the thyroid hormone with the greatest effect inside the cells. Your thyroid produces only a small share of the T3 itself, and most of it is created elsewhere in the body by conversion of T4. The laboratory measures free T3, the fraction that is not bound to protein and is therefore available to the cells.
What is the difference between T3 and free T3?
Total T3 is all the T3 in your blood, including the part bound to proteins. Free T3, also called FT3, is only the unbound part that the cells can use. Because the amount of binding proteins can vary, for example during pregnancy or when using the contraceptive pill, free T3 gives a more stable picture. That is why the laboratory usually measures free T3.
How much does a free T3 blood test cost?
A single free T3 test costs €55. On top of that you pay a one-off €29 blood draw fee per order, which is waived from €225. Testing free T3 on its own therefore comes to €84 all-in.
In the Health Guide
Read more about this value
Where do you give blood?
You can give blood at collection points across the Netherlands, usually without an appointment.
Find a collection point near you or read how a blood draw works.
Ready for insight into your Free T3 level?
You receive your Free T3 level including personal explanation from a doctor, typically within 2 working days. No referral required.
Order Free T3 →Want to test multiple markers at once?
Combine Free T3 with other markers via Bloodworks Select. The blood draw and collection fee is charged once per order, and is waived from €225.
55,-
+ €29 blood draw fee+ €29 blood draw fee per order