Blood draw
Total T4
Total thyroxine (T4); for thyroid function assessment.
€ 32,00
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Thyroid · Blood
Total T4
The total amount of thyroxine in the blood, bound and free; also depends on transport protein, which is why free T4 usually says more.
At a glance
- Measured in blood
- In 1 test panel
- Check the preparation
Explanation
Total T4: what is measured?
The thyroid mainly makes thyroxine (T4). Almost all T4 in the blood is bound to transport proteins, mainly thyroxine-binding globulin (TBG); only a very small part is free and active. This test measures total T4: the bound and the free part together. This means the result depends not only on the thyroid, but also on the amount of transport protein in the blood.
What can it detect?
- Overactive or underactive thyroid, together with TSH (hyperthyroidism or hypothyroidism)
- A change in the amount of transport protein (TBG), for example due to the pill or a pregnancy
- Limited: a check of an unexpected free T4, for example when a measurement interference is suspected
Result
What does the result mean?
Total T4 too high
A raised total T4 together with a low TSH fits an overactive thyroid. With a normal TSH, a raised value is often caused by more transport protein in the blood, for example when using the contraceptive pill or during pregnancy; the free, active T4 is then normal. With use of levothyroxine, the value can also be higher shortly after taking it.
Total T4 too low
A low total T4 together with a raised TSH fits an underactive thyroid. With a normal TSH, a low value is often caused by less transport protein, for example in severe illness or malnutrition, or by an inborn shortage of TBG; the latter has no consequences for health. Rarely, it is a problem of the pituitary gland, which then makes too little TSH.
After an abnormal result
The GP assesses thyroid function with TSH and free T4. An abnormal total T4 with a normal TSH and free T4 is usually no reason for further tests.
Reliability
How reliable is the test?
Total T4 moves along with the amount of transport protein; according to the Dutch Society for Clinical Chemistry (NVKC), free T4 therefore shows better how much active hormone there is. Thyroid function is determined with TSH and free T4 (Dutch GP guideline (NHG) on thyroid disorders). Total T4 is mainly useful as a check of an unexpectedly high free T4 due to heparin (guideline on thyroid function disorders). The value can only be interpreted alongside TSH.
What can affect the result?
- The contraceptive pill and pregnancy raise the transport protein and with it total T4, without the thyroid working faster.
- Severe illness, malnutrition and an inborn TBG shortage lower total T4, without the thyroid working more slowly.
- With use of levothyroxine, the value is higher shortly after taking it; preferably take the tablet after the blood draw.
- Reference values differ by laboratory and measurement method.
Preparation
- Take your thyroid medication after the blood test.
- Do you take biotin (vitamin B8, often in hair and nail supplements)? Stop at least 2 days before the sample is taken, or 4 days for a high dose.
Who it is for
Who is the test useful for?
Useful for
For thyroid function in adults, TSH and free T4 say more than total T4, because total T4 also changes with the amount of transport protein. Total T4 can at most give additional information alongside those values. If you use the pill or are pregnant, take into account a higher result without thyroid disease.
How common is it?
An underactive thyroid affects about 3 to 4 in 100 adults in the Netherlands and an overactive thyroid about 1 in 100, both more often women and increasing with age. According to the Dutch Society for Clinical Chemistry (NVKC), a partial inborn shortage of TBG occurs in about 1 in 200 people.
NIVEL primary care registrations, the NHG guideline on thyroid disorders and NVKC (allesovertesten.nl)
Drawbacks and risks
- A raised total T4 due to the pill or a pregnancy can wrongly be seen as an overactive thyroid and lead to unnecessary worry or further tests.
Get tested
Total T4: test panels
Total T4 is part of these test panels. You order online and have your blood drawn at a collection point or collect the sample at home.
Evidence
Sources
This explanation is based on Dutch guidelines and independent information. It is general information, not a diagnosis. Discuss an abnormal result with your GP.
More about risks and resultsMore explanations
Other tests: Thyroid
- Anti-Tg
- Anti-TPO
- Free T3
- Free T4
- Reverse T3
- T3 (total)
- Thyroglobulin
- Thyroxine-binding globulin
- TSH
- TSH receptor antibodies (TRAb)