FT3 test

fT3 (free triiodothyronine) test

Free triiodothyronine is one of the main thyroid hormones. Its action is crucial for proper functioning of the entire body.

Free triiodothyronine (fT3) and free thyroxine (fT4) are hormones produced by the thyroid gland. They control the functions of most organs, support metabolic processes and the functioning of the central nervous system. The thyroid gland primarily produces thyroxine (FT3) – 85%. Triiodothyronine is also produced by the thyroid gland, but approximately 80% of this hormone comes from thyroxine conversion, which occurs in the cells of the body. The remaining part is secreted directly by the thyroid gland.

fT3 concentration is determined for detailed diagnostics of thyroid diseases.

The main indication to perform this test is the suspicion of hyperthyroidism, when low TSH concentration levels are accompanied by unremarkable fT4 concentrations. It is not recommended to determine triiodothyronine in order to adjust the dosage of thyroxine in patients with hypothyroidism.

Determining the concentration of fT4 is more reliable than determining fT3, because the concentration of triiodothyronine primarily depends on extra-thyroidal factors.

FT3 action

The functions of the fT3 (free triiodothyronine) hormone:

• metabolic regulation
• supporting the development of the nervous system in infants
• supporting the circulatory system

Thyroid hormones are necessary for proper functioning of the body. Therefore, it is very important to diagnose any suspected diseases of this gland.

Increased fT3 concentration levels with a simultaneous decrease in TSH concentration below the norm may indicate the presence of hyperthyroidism. Low fT3 levels with elevated TSH concentration may be a sign of hypothyroidism.

When should the FT3 test be done?

The thyroid hormone concentration test is indicated for individuals with suspected hypo- or hyperthyroidism.

The test results should always be interpreted along with the assessment of TSH concentration.

The presence of the disease depends on the concentration of thyroid hormones. Abnormal conversion of the fT4 hormone into fT3 is caused by such factors as:

• poor diet (deficiencies of iron, zinc, selenium, as well as vitamins A, B and B12)
• an increase or decrease in body weight
• diabetes
• the presence of excessive levels of mercury, lead or pesticides in the body
• alcohol abuse
• surgical procedures
• infections
• psychiatric diseases
• circulatory failure

Lack of hormonal balance may lead to excessive concentration of fT4 compared to fT3, and disturbances of fT4 to fT3 conversion lead to the emergence of hypothyroidism symptoms.

FT3 – preparation for the test

Patients are allowed to drink a small amount of water before the blood test.

It is recommended that the blood sample be collected in the morning hours (07:00 am – 10:00 am).

• The test involves the collection of blood according to the standard procedure of blood collection (disinfecting the skin, putting on a tourniquet and applying pressure in the collection site – after the blood collection procedure is over, it is necessary to compress the injection site with a gauze square for 5 minutes, with the arm kept straight).
• The blood collection takes from a few to a few dozen seconds.
• No special preparation is required for the test.
• Blood should be collected in the morning.

FT3 results

• Test results are usually available within 1 working day.
• It is possible to collect them at the reception desk of a selected centre.
• The results can be accessed online. For this to be possible, it is necessary to collect a password at the reception desk of a selected centre.