Thyroid
Totaal 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.
Everything about this testWhat it measures
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.
- 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
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.
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.
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.
If your result is abnormal
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
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.
Limitations
- 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.
How common is this
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.
Who is this 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.
Downsides 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.
Sources
Always discuss an abnormal result with your GP. If a result is strongly abnormal, we try to reach you as soon as possible. Read your result yourself as well and contact your GP if in doubt. A test does not make a diagnosis. Read more about risks and results