Free triiodothyronine, abbreviated to fT3, is one of the hormones produced by the thyroid gland. This hormone is characterized by high biological activity and is one of the parameters for the assessment of thyroid function. What does too high or too low fT3 concentration mean? Check when the fT3 test should be performed and how to interpret the results.

FT3 , orfree triiodothyronineis a thyroid hormone produced by the follicle cells of the thyroid gland.The fT3 testis usually performed when thyroid diseases are suspected. It enables the determination of the concentration of the free fraction of triiodothyronine (fT3).

What is fT3?

Triiodothyronine (T3) is a hormone produced by thyroid follicular cells together with thyroxine (T4), from which it has several times higher biological activity. Additionally, T3 is formed in peripheral tissues by enzymatic detachment of the iodine atom from the T4 molecule.

T3 in the blood occurs mainly in the form bound to plasma proteins, e.g. globulin. The rest of the hormone occurs as free triiodothyronine, or fT3.

It is the determination of fT3 in the blood that is of diagnostic importance in the assessment of thyroid function.

Test fT3 - indications

The indication for fT3 testing is suspected hypothyroidism or hyperthyroidism, preferably together with free T4 (fT4) and thyrotropin (TSH) testing. The fT3 test is also used to predict recurrence of Graves' disease or suspect amidarone-induced hyperthyroidism.

Another indication is the suspicion of T3-thyrotoxicosis, i.e. when the patient has low TSH levels and normal fT4 with symptoms of hyperthyroidism.

fT3 test - what is the test and how to prepare?

To perform the fT3 test, venous blood is needed from the elbow bend. The blood is collected on the so-called clot, i.e. with an activator that accelerates the blood clotting process.

The test should be performed on an empty stomach, preferably between 6:30 and 9:00.

Test fT3 - laboratory standards

Laboratory standards for fT3 will depend, among others, on on the method used in the laboratory. For the determination with the ECLIA method, the normal range for an adult is 2.8-7.1 pmol / l.

But the resulttests should always be compared to the standard value placed on the result.

Test fT3 - what does it mean too high or too low?

High fT3 levels may indicate primary or secondary hyperthyroidism. Low fT3 levels may indicate primary or secondary hypothyroidism.

Low level of maybe fT3 is also observed in the so-called low T3 syndrome, which is caused by chronic disease (e.g. diabetes), medication (e.g. steroids), or intense exercise.

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