The T3 test is requested by the doctor after altered TSH or T4 hormone results or when the person presents signs and symptoms of hyperthyroidism, such as nervousness, weight loss, irritability and nausea, for example.
The TSH hormone is responsible for stimulating the production of T4, mainly, which is metabolized in the liver with the aim of giving rise to its most active form, T3. Although most of T3 is derived from T4, the thyroid also produces this hormone, but in smaller quantities.
It is not necessary to fast to take the test, however, some medications can interfere with the test results, such as thyroid medication and contraceptives, for example. Therefore, it is important to inform the doctor so that guidance can be given regarding the safe suspension of the medication to carry out the exam.
What is it for
The T3 test is requested when the TSH and T4 test results are altered or when the person presents symptoms of hyperthyroidism. As it is a hormone that is normally found in low concentrations in the blood, the measurement of T3 alone is not widely used to assess thyroid function, and is normally requested when the diagnosis of thyroid abnormalities is confirmed or together with TSH and T4. . Find out about other tests that evaluate the thyroid.
In addition to being useful in helping to diagnose hyperthyroidism, the T3 test can also be requested to help identify the cause of hyperthyroidism, such as Graves’ disease, for example, and is normally requested together with the measurement of thyroid autoantibodies.
How is done
The T3 exam is carried out by evaluating a blood sample, which is analyzed in the laboratory. In the evaluation, the concentrations of total T3 and free T3 can be verified, which corresponds to T3 bound to proteins, which represents around 0.3% of total T3.
It is not necessary to fast to take this test, however it is important to indicate whether any medication is being used, as it may interfere with the result.
Reference value
The reference value of T3 total It is between 80 and 180 ng/dL e of Free T3 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:
- T3 alto: It usually confirms the diagnosis of hyperthyroidism, being indicative of Graves’ disease, mainly;
- T3 low: It may indicate Hashimoto’s thyroiditis, neonatal hypothyroidism or secondary hypothyroidism, requiring additional tests to confirm the diagnosis.
The results of the T3 test, as well as the 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 the cause of this dysfunction. Therefore, the doctor may request more specific tests to identify the cause of hypo or hyperthyroidism, such as blood counts, immunological and imaging tests.
What is reverse T3?
Reverse T3 is the inactive form of the hormone derived from the conversion of T4. The measurement of reverse T3 is rarely requested and is only indicated for patients with serious diseases involving the thyroid, with decreased levels of T3 and T4 being detected, but elevated levels of reverse T3. Furthermore, reverse T3 may be elevated in situations of chronic stress, HIV infection and renal failure.
The reverse T3 reference value for newborns is between 600 and 2500 ng/mL e from the 7th day of life, between 90 and 350 ng/mLwhich may vary between laboratories.
