Thyroid
T3
The active thyroid hormone, measured as the total amount; mainly an addition when an overactive thyroid is suspected.
Everything about this testWhat it measures
Triiodothyronine (T3) is the active thyroid hormone. The thyroid itself makes only a small part of it; most T3 is formed in the tissues from T4. Almost all T3 in the blood is bound to proteins and only a very small part is free. This test measures total T3: the bound and the free part together. This means the result also depends on the amount of binding proteins in the blood.
- Overactive thyroid (hyperthyroidism), including the form in which mainly T3 is raised (T3 toxicosis)
- Limited: additional information if TSH and free T4 do not fit the symptoms
High
A raised T3 together with a low TSH fits an overactive thyroid, for example due to Graves' disease or a thyroid nodule that makes hormone independently. If free T4 is normal, this is called T3 toxicosis. Because the test also measures the bound hormone, total T3 can come out higher without the thyroid being overactive, for example when using the contraceptive pill or during pregnancy, because there is then more binding protein in the blood. Medication with T3 (liothyronine) also raises the value.
Low
A low T3 is usually not a thyroid disease. During illness, fasting, malnutrition and at older ages, the body converts less T4 into T3; medicines such as amiodarone, propranolol and corticosteroids also slow this conversion. Less binding protein in the blood, for example in severe illness or an inborn variation, also gives a lower value. With an underactive thyroid, T3 can be low, but it often stays normal for a long time; TSH and free T4 show this more reliably.
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 assesses T3 together with TSH and free T4. An abnormal T3 with a normal TSH is usually not investigated further or is first repeated; with a low TSH and a raised T3, tests follow to find the cause of an overactive thyroid, such as TSH receptor antibodies.
Reliability
TSH and free T4 say more about thyroid function than T3. According to the guideline on thyroid function disorders, (free) T3 mainly adds something when TSH is unmeasurably low with a normal free T4, to show T3 toxicosis. Because total T3 also measures the bound hormone, the result partly depends on the amount of binding protein. The value can only be interpreted alongside TSH and free T4.
Limitations
- Total T3 changes with the amount of binding protein: higher with the contraceptive pill and pregnancy, lower with severe illness and malnutrition.
- T3 falls with any severe or long-lasting illness, with fasting and with some medicines, without the thyroid being diseased.
- There is no agreed reference interval; normal values differ by laboratory and by age.
- With an underactive thyroid, T3 stays normal for a long time; the test is unsuitable there as the first or only test.
How common is this
An overactive thyroid affects about 1 in 100 adults in the Netherlands, more often women. According to the NHG guideline, Graves' disease is the cause in 70 to 80 in 100 of them. An abnormal T3 with a normal TSH is more common and usually has a cause other than a thyroid disease.
Who is this useful for
Useful as an addition when an overactive thyroid is suspected, especially if TSH is low and free T4 is normal. With tiredness or other symptoms and a normal TSH, T3 adds little. If you use the pill or are pregnant, take into account a higher result without thyroid disease.
Downsides and risks
- A slightly abnormal total T3 due to the pill, a pregnancy or another illness can wrongly be seen as a thyroid problem.
Sources
- NHG-Standaard Schildklieraandoeningen (M31)
- FMS Richtlijnendatabase: Schildklierfunctiestoornissen, module Biochemische bepalingen
- NVKC, allesovertesten.nl: T3
- NVKC, allesovertesten.nl: TBG
- Thuisarts.nl: Te snel werkende schildklier
- NIVEL Zorgregistraties Eerste Lijn: jaarprevalentie schildklieraandoeningen in de huisartsenpraktijk
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