- What are T4 and fT4?
- Test fT4 - indications
- fT4 test - what is the test and how to prepare?
- Test fT4 - laboratory standards
- fT4 test - what does it mean too high or too low?
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Free thyroxine, or fT4, is one of the thyroid hormones. Testing the level of fT4 in the blood is an important parameter for assessing thyroid function. How to prepare for the fT4 test? What does fT4 level too low or too high mean?
Thyroxin test -is a test that aims to determine the level of thyroxine in the blood, and thus - to assess the thyroid function. Most of the circulating thyroxine in the blood is bound to specific proteins, and only about 1 percent. it circulates in the form of a free hormone that affects the functioning of the body. Therefore, during the test, the level of the so-calledfree thyroxine - fT4 .
What are T4 and fT4?
Thyroxine (T4) is a hormone produced by thyroid follicular cells together with triiodothyronine (T3). T4 is formed from the amino acid tyrosine and iodine with the participation of specialized enzymes.
In the blood, this hormone occurs mainly in the form associated with plasma proteins, e.g. globulin. The rest of the hormone is in the form of free thyroxine, abbreviated to fT4 (from the English word "free"). The most common in clinical practice is the determination of fT4 in order to avoid errors resulting from changes in the concentration of plasma proteins, which are affected, for example, by pregnancy, certain medications.
In peripheral tissues, fT4 is transformed into T3. Receptors for thyroid hormones are found in practically every tissue of the body, hence the biological effect of thyroid hormones is multidirectional and affects several organs at the same time.
The level of fT4 is tightly controlled by the thyroid stimulating hormone (TSH), which is secreted by the pituitary gland.
When fT4 levels drop, TSH levels rise to simulate its production by the thyroid gland. On the other hand, when the fT4 level increases, the TSH level decreases and the thyroid gland inhibits the production of the thyroid hormone.
Test fT4 - indications
The indication for fT4 testing is suspected hypothyroidism or hyperthyroidism. The hormone fT4 is often tested along with fT3 and TSH. The fT4 test is also helpful in monitoring the effectiveness of the treatment of thyroid dysfunction.
fT4 test - what is the test and how to prepare?
To perform the fT4 test, you need venous blood taken from the elbow bend. Blood is collected on the so-called clot, that is, with an activatoraccelerating the blood clotting process. The test should be performed on an empty stomach, preferably between 6:30 am and 9:00 am.
Test fT4 - laboratory standards
Laboratory standards for fT4 will depend, among others, on on the method used in the laboratory. For the determination by the ECLIA method, the normal range for an adult is 9-22 pmol / l. Therefore, the test result should always be compared to the standard shown on the result.
The normal range for fT4 is also dependent on age and trimester of pregnancy:
- newborn 22-49 pmol / l
- child 5 years old 9-20 pmol / l
- child 14 years old 8-17 pmol / l
- pregnant in the first trimester 9-26 pmol / l
- pregnant in the second and third trimesters 6.5-21 pmol / l
fT4 test - what does it mean too high or too low?
High fT4 levels may indicate primary or secondary hyperthyroidism. Low fT4 levels may indicate primary or secondary hypothyroidism.
The diagnosis of thyroid dysfunction should be additionally based on TSH levels and fT3 tests. If normal fT4 levels are associated with low or high TSH levels, this may indicate subclinical (latent) hypothyroidism or hyperthyroidism.
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