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Diagnostic Tests 2023

T3 and T4: what they are, what they are for and when the exam é indicated

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T3 and T4: what they are, what they are for and when the exam é indicated
T3 and T4: what they are, what they are for and when the exam é indicated

T3 and T4 are hormones produced by the thyroid gland, under stimulation of the TSH hormone, which is produced by the pituitary gland, and which participate in several processes in the body, mainly related to metabolism and energy supply for the proper functioning of the body. body.

The dosage of these hormones is indicated by the endocrinologist or general practitioner with the aim of evaluating the person's general he alth or investigating the possible cause of some symptoms that may be related to thyroid malfunction, such as excessive tiredness, fall hair loss, difficulty losing weight and loss of appetite, for example.


What they are for

T3 and T4 hormones are produced by the thyroid gland and regulate various processes in the body, mainly related to cellular metabolism. Some of the main functions of T3 and T4 in the body are:

  • Normal development of brain tissues;
  • Metabolism of fats, carbohydrates and proteins;
  • Regulation of heartbeat;
  • Stimulation of cellular respiration;
  • Menstrual cycle regulation.

T4 is produced by the thyroid and remains bound to proteins so that it is transported in the bloodstream to various organs and thus can perform its function. However, to have function, T4 is separated from the protein, becoming active and becoming known as free T4. Learn more about T4.

In the liver, the T4 produced is metabolized to give rise to another active form, which is T3. Although T3 is primarily derived from T4, the thyroid also produces these hormones in smaller amounts. See more information about T3.


When the exam is indicated

The dosage of T3 and T4 is indicated when there are signs and symptoms indicating that the thyroid is not working properly, which may be indicative of hypo or hyperthyroidism, Graves' disease or Hashimoto's thyroiditis, for example.

Furthermore, performing this examination can also be routinely recommended in order to assess the person's general he alth, in the investigation of female infertility and in the suspicion of thyroid cancer.

Thus, some of the signs and symptoms that may be indicative of thyroid disorder and that the dosage of T3 and T4 levels is recommended are:

  • Difficulty losing weight or gaining weight easily and quickly;
  • Fast weight loss;
  • Excessive fatigue;
  • Weakness;
  • Increased appetite;
  • Hair loss, dry skin and brittle nails;
  • Swelling;
  • Menstrual cycle change;
  • Change of heart rate.

In addition to the dosage of T3 and T4, other tests are usually requested to help confirm the diagnosis, being mainly requested to carry out the dosage of the TSH hormone and antibodies, in addition to the possibility of performing thyroid ultrasound. Learn more about the tests indicated to evaluate the thyroid.

How to understand the result

The results of the T3 and T4 exams must be evaluated by the endocrinologist, general practitioner or physician who indicated the exam, and the result of other exams that evaluate the thyroid, the person's age and the general he alth status. In general, the levels of T3 and T4 considered normal are:

  • T3 total: 80 and 180 ng/dL;
  • Free T3: 2.5 - 4.0 ng/dL;
  • T4 total: 4.5 - 12.6 µg/dL;
  • FreeT4: 0.9 - 1.8 ng/dL.

So, according to the values of T3 and T4 it is possible to know if the thyroid is working properly. Normally, T3 and T4 values above the reference value are indicative of hyperthyroidism, while lower values are indicative of hypothyroidism, however other tests are necessary to confirm the result.

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