The T3 exam is ordered by the doctor after altered TSH or hormone T4 results or when the person has signs and symptoms of hyperthyroidism, such as nervousness, weight loss, irritability and nausea, for example.
The hormone TSH is responsible for stimulating the production of T4, mainly, which is metabolized in the liver in order to give rise to its most active form, T3. Although most T3 is derived from T4, the thyroid also produces this hormone, but in smaller amounts.
It is not necessary to be fasted to perform the test, however, some medications can interfere with the test result, such as thyroid and contraceptive drugs, for example. Therefore, it is important to communicate to the doctor so that guidance can be given regarding the safe suspension of the drug to perform the test.
What is it for
The T3 exam is ordered when the results of the TSH and T4 exam are altered or when the person has symptoms of hyperthyroidism. Because it is a hormone that is normally found in low blood concentrations, the T3-only dosage is not widely used to assess thyroid function, being normally requested when there is confirmation of the diagnosis of thyroid alteration or together with TSH and T4. Learn about other tests that evaluate the thyroid.
In addition to being useful to aid in the diagnosis of hyperthyroidism, the T3 test can also be ordered to help identify the cause of hyperthyroidism, such as Graves' disease, for example, and is usually ordered together with the measurement of thyroid autoantibodies.
The test is done from a blood sample sent to the laboratory, in which the concentration of total T3 and free T3 is measured, which corresponds to only 0.3% of the total T3, being thus more found in its protein conjugated form.. The reference value for total T3 is between 80 and 180 ng / dL and for free T3 it is between 2.5 - 4.0 ng / dL, which may vary according to the laboratory.
How to understand the result
T3 values vary according to the person's health, and may be increased, decreased or normal:
- High T3: Usually confirms the diagnosis of hyperthyroidism, being indicative of Graves' disease, mainly; Low T3: May indicate Hashimoto's thyroiditis, neonatal hypothyroidism or secondary hypothyroidism, requiring additional tests to confirm the diagnosis.
The results of the T3 exam, as well as that of T4 and TSH, only indicate that there is some change in the production of hormones by the thyroid, and it is not possible to determine what is the cause of this dysfunction. Therefore, the doctor may order more specific tests to identify the cause of hypo or hyperthyroidism, such as blood count, immunological and imaging tests.
What is reverse T3?
Reverse T3 is the inactive form of the hormone derived from T4 conversion. The measurement of T3 reverse is little requested, being indicated only for patients with severe diseases involving the thyroid, with reduced levels of T3 and T4, but high levels of reverse T3 being detected. In addition, reverse T3 may be elevated in situations of chronic stress, HIV infection and renal failure.
The reference value for reverse T3 for newborns is between 600 and 2500 ng / mL and from the 7th day of life on, between 90 and 350 ng / mL, which may vary between laboratories.
