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Diagnostic Tests 2023

Exam T3: what is it for and how to understand the results

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Exam T3: what is it for and how to understand the results
Exam T3: what is it for and how to understand the results

The T3 exam is requested by the doctor after abnormal TSH or T4 hormone results or when the person has 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 in order to give rise to its most active form, T3. Although most of T3 is derived from T4, the thyroid also produces this hormone, but in smaller amounts.

It is not necessary to fast for the exam, however, some medications can interfere with the exam result, 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 perform the exam.


What is it for

The T3 test is ordered when the TSH and T4 test results are abnormal or when the person has symptoms of hyperthyroidism. As it is a hormone that is normally found in low concentrations in the blood, the dosage of T3 alone is not widely used to assess thyroid function, being usually requested when there is confirmation of the diagnosis of thyroid alteration or together with TSH and T4.. Find out about other tests that evaluate the thyroid.

In addition to being useful to aid in the diagnosis of hyperthyroidism, the T3 exam can also be ordered to help identify the cause of hyperthyroidism, such as Graves' disease, for example, being normally requested along with the autopsy dosage. -thyroid antibodies.

How it's done

The T3 exam is performed from the evaluation of 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 the T3 bound to proteins, which represents about 0.3% of the total T3.

To perform this exam it is not necessary to perform fasting, however it is important to indicate if any medication is being used, since it can interfere with the result.

Reference value

The reference value of T3 total is between 80 and 180 ng/dL and of T3 free is between 2.5 - 4.0 ng/dL,may vary by laboratory.

How to understand the result

T3 values vary according to the person's he alth, and may be increased, decreased or normal:

  • High T3: Usually confirms the diagnosis of hyperthyroidism, being mainly indicative of Graves' disease;
  • Low T3: May indicate Hashimoto's thyroiditis, neonatal hypothyroidism or secondary hypothyroidism, requiring additional tests to confirm the diagnosis.

The result of the T3 exam, as well as the T4 and TSH, only indicate that there is some alteration 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 count, immunological and imaging tests.

What is reverse T3?

Reverse T3 is the inactive form of the hormone derived from the conversion of T4. The dosage of reverse T3 is rarely requested, being indicated only for patients with severe diseases involving the thyroid, with decreased levels of T3 and T4 being detected, but elevated levels of reverse T3. In addition, reverse T3 may be elevated in situations of chronic stress, HIV infection and renal failure.

Reverse T3 reference value for newborns is between 600 and 2500 ng/mL and from the 7th day of life, between 90 and 350 ng/mL, which may vary between laboratories.

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