Thyroid
Vrij T3
The most active thyroid hormone; mainly of value when an overactive thyroid is suspected, less so with an underactive one.
Everything about this testWhat it measures
Triiodothyronine (T3) is the active form of thyroid hormone. The body makes most of it not in the thyroid but in the tissues, by conversion from T4. Free T3 (FT3) is the small part that is not bound to protein. The value reacts strongly to other illnesses, fasting and medicines, and is therefore less stable than TSH and free T4.
- Overactive thyroid (hyperthyroidism), including the form in which only T3 is raised (T3 toxicosis)
- Monitoring during treatment of an overactive thyroid
- Limited: additional information with an underactive thyroid, as free T3 only falls late in that case
High
A raised free T3 together with a low TSH fits an overactive thyroid, for example due to Graves' disease or an overactive thyroid nodule. If free T4 is normal, this is called T3 toxicosis. Medication containing T3 (liothyronine, desiccated thyroid preparations) or biotin supplements can also raise the value.
Low
A low free T3 is usually not a thyroid disease. During illness, fever, a strict diet or malnutrition, the body converts less T4 into T3; the value is also lower at older age and with medicines such as beta blockers, amiodarone and corticosteroids. With an underactive thyroid, free T3 only falls at a late stage, so a normal value does not rule out an underactive thyroid.
Preparation
- 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 always assesses free T3 together with TSH and free T4. An abnormal free T3 with a normal TSH is usually repeated, preferably at a time without another illness or diet, before any conclusions are drawn.
Reliability
Free T3 on its own rarely makes a diagnosis. As a first test for thyroid symptoms, free T3 says little; the value mainly adds something when an overactive thyroid with a normal free T4 is suspected, and when monitoring that treatment. The value can only be interpreted alongside TSH and free T4.
Limitations
- Free T3 falls with any serious or long-lasting illness, with fasting and with low-carbohydrate diets, without the thyroid itself being ill.
- Biotin supplements can disturb the measurement; stop biotin at least two days before the blood draw.
- Measuring free T3 is technically more difficult than measuring free T4 and differs more between laboratories.
- With an underactive thyroid, free T3 stays normal for a long time; the test is unsuitable there as the first or only measurement.
How common is this
An overactive thyroid affects about 1 in 100 adults in the Netherlands, more often women. In a small proportion of them only T3 is raised (T3 toxicosis). A low free T3 with a normal TSH is much more common and is usually a result of another illness or of a diet.
Who is this useful for
Useful when an overactive thyroid is suspected, especially if TSH is low and free T4 is normal, and for monitoring its treatment. With tiredness and a normal TSH, free T3 adds little. A low value is not proof of an underactive thyroid.
Downsides and risks
- A low free T3 is often wrongly explained online as an underactive thyroid or as a reason for T3 medication; with a normal TSH and free T4, this is not supported by evidence.
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