Thyroid · Blood

Free T3

The most active thyroid hormone; mainly of value when an overactive thyroid is suspected, less so with an underactive one.

At a glance

Measured in blood
In 8 test panels
Check the preparation

Explanation

Free T3: what is measured?

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.

What can it detect?

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

Result

What does the result mean?

Free T3 too 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.

Free T3 too 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.

After an abnormal result

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

How reliable is the test?

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.

What can affect the result?

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.

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.

Who it is for

Who is the test useful for?

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.

How common is it?

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.

NIVEL primary care registrations and the NHG guideline on thyroid disorders

Drawbacks 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.

Get tested

Free T3: test panels

Free T3 is part of these test panels. You order online and have your blood drawn at a collection point or collect the sample at home.

Blood draw
rT3 and FT3
Reverse T3 and free T3; supplementary thyroid diagnostics.
€ 85,00
Blood draw
Thyroid Test (TSH, Free T4, FT3)
TSH plus free T4 and FT3 for assessment of thyroid function.
€ 60,00
Blood draw
Free T3 (FT3)
Free T3; for thyroid function.
€ 32,00
Blood draw
Free T4 and Free T3
Free T4 and FT3; for thyroid function.
€ 41,00
Fingerstick
Thyroid Panel: TSH, Free T4, and FT3
Combination of TSH, free T4, and FT3.
€ 60,00
Fingerstick
Free T4 and Free T3
Free T4 and Free T3; concerning thyroid function.
€ 41,00
Fingerstick
Free T3 (FT3)
Free T3; for thyroid function.
€ 32,00
Blood draw
Supplemental: Testosterone Replacement Therapy (TRT)
Complete hormonal analysis with testosterone, DHEA-S, thyroid, blood count, liver function, kidney function, lipids, glucose, HbA1c, insulin, hs-CRP, vitamin D and ferritin.
€ 335,00

Free T3