Thyroid
Vrij T4
The thyroid hormone thyroxine, measured as the free part the body can use; shows how much hormone the thyroid actually makes.
Everything about this testWhat it measures
The thyroid mainly makes thyroxine (T4). Most of it is bound to proteins in the blood; only the small free part (free T4 or FT4) is active in the tissues. Free T4 is almost always assessed together with TSH: TSH shows whether the control signal is right, free T4 shows whether the thyroid actually responds by making enough or too much hormone.
- Underactive thyroid (hypothyroidism): low free T4 with a raised TSH
- Overactive thyroid (hyperthyroidism): raised free T4 with a low TSH
- Checking the dose when using thyroid medication
High
A raised free T4 together with a low TSH fits an overactive thyroid, for example due to Graves' disease, an overactive thyroid nodule or thyroid inflammation. When using levothyroxine, it points to too high a dose or to a blood draw shortly after taking it. A raised free T4 with a normal TSH is unusual and is more often due to biotin supplements, medicines such as amiodarone or heparin, or interference with the measurement than to a thyroid disease.
Low
A low free T4 together with a raised TSH fits an underactive thyroid. If TSH is normal or low, the cause usually does not lie in the thyroid itself: we see this with a serious other illness, malnutrition, some medicines (including corticosteroids and antiepileptics) or, rarely, a problem of the pituitary gland.
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 free T4 together with TSH. With a combination that points to a thyroid disorder, a repeat measurement usually follows after a few weeks and, if needed, testing into the cause, such as thyroid antibodies or an ultrasound.
Reliability
Free T4 is the confirmation test after an abnormal TSH, not the first screening test: in an early thyroid disorder, TSH is already abnormal while free T4 is still normal (subclinical). The combination of TSH plus free T4 reliably establishes the direction of a disorder; the cause requires additional testing.
Limitations
- Biotin (vitamin B8 in high doses, for example in hair and nail supplements) can make free T4 measure falsely high; stop biotin at least two days before the blood draw.
- When using levothyroxine, free T4 is temporarily higher a few hours after taking it; preferably take the tablet after the blood draw.
- During pregnancy and with serious illness, different reference values apply or the result is harder to interpret.
- Reference values differ per laboratory and measurement method; only compare results from the same laboratory.
How common is this
An underactive thyroid affects about 3 to 4 in 100 adults in the Netherlands, more often women and older people; an overactive thyroid about 1 in 100. An abnormal free T4 with a normal TSH is rare.
Who is this useful for
Useful together with TSH with symptoms that fit a thyroid disorder, with a previously abnormal TSH, with a known thyroid condition and for monitoring thyroid medication. Apart from TSH, free T4 has little value; in adults without symptoms or risk factors, routine measurement is not needed.
Downsides and risks
- Without TSH alongside it, a single free T4 value is hard to interpret, and a normal result can give false reassurance with an early disorder.
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