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Diagnostic Tests 2023

T4 free and total: what são and what they are for and values

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T4 free and total: what são and what they are for and values
T4 free and total: what são and what they are for and values

T4 is a hormone produced by the thyroid that is responsible for supporting metabolism, providing the energy necessary for the correct functioning of the body. T4 is almost totally conjugated to proteins so that it can be transported in the bloodstream to various organs and can perform its function.

Free T4 corresponds to the concentration of T4 that is not bound to proteins, exerting its function in the body. While total T4 corresponds to the concentration of T4 bound and not bound to proteins.

This exam can be recommended by the doctor in routine exams, but it is more indicated when the person has symptoms of hypo or hyperthyroidism, for example, or when there is an altered TSH result. See what the TSH exam is for and the reference values.


What is it for

T4 is a very important hormone for metabolism and, therefore, its dosage is done with the aim of evaluating whether the thyroid produces a normal and sufficient amount of hormones to provide energy for the body's metabolic activities.

Less than 1% of T4 is in the free form, and it is this form that is metabolically active, that is, that has function. Protein-bound T4 has no activity, it is only transported in the bloodstream to the organs, and when necessary, is separated from the protein to have activity.

Total T4 corresponds to the total amount of hormone produced, being evaluated both the amount that is conjugated to proteins and the amount that is free circulating in the blood. However, the dosage of total T4 can be somewhat non-specific, as there may be interference with the proteins that the hormone can bind to.

Free T4, on the other hand, is more specific, sensitive and allows for a better assessment of the thyroid, as only the amount of hormone that is functional and active in the body is measured

How the exam is done

The test is performed with a blood sample and no preparation is required before performing it. However, if the person is using any medication that interferes with the thyroid, they should inform the doctor so that this is taken into account at the time of analysis.

Reference value

The normal values of freeT4 are between 0, 9 - 1, 8 ng/dL, while the normal values of Total T4 varies according to age:

Age Normal values of total T4
1st week of life 15 µg/dL
Up to the 1st month 8, 2 - 16.6 µg/dL
Between 1 and 12 months of life 7, 2 - 15.6 µg/dL
Between 1 and 5 years old 7, 3 - 15 µg/dL
Between 5 and 12 years old 6, 4 - 13, 3 µg/dL
Over 12 years old 4, 5 - 12.6 µg/dL

High or low T4 values can indicate hypo or hyperthyroidism, thyroid cancer, thyroiditis, goiter and female infertility, for example. In addition, decreased free T4 values may indicate malnutrition or Hashimoto's thyroiditis, for example, which is an autoimmune disease characterized by thyroid inflammation leading to hyperthyroidism followed by hypothyroidism.

When to do

The T4 test is usually requested by the endocrinologist in situations such as:

  • Altered TSH test result;
  • Weakness, decreased metabolism and tiredness, which may be indicative of hypothyroidism;
  • Nervousness, increased metabolism, increased appetite, which may indicate hyperthyroidism;
  • Suspected thyroid cancer;
  • Investigation of the cause of female infertility.

Based on the evaluation of the test results and the person's symptoms, the endocrinologist can define the diagnosis and the best form of treatment, thus normalizing T4 levels. Find out about other essential tests to evaluate the thyroid.

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