T4 is a hormone produced by the thyroid that is responsible for supporting metabolism, providing the energy necessary for the body to function correctly. T4 is almost completely 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 test may be recommended by the doctor during routine exams, but it is more recommended when the person presents symptoms of hypo or hyperthyroidism, for example, or when there is an altered TSH result. See what the TSH test is for and the reference values.
What is it for
T4 is a very important hormone for metabolism and, therefore, its dosage is made 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 found in free form, and it is this form that is metabolically active, that is, it has a function. T4 bound to proteins has no activity, it is only transported in the bloodstream to the organs, and when necessary, it is separated from the protein to have activity.
Total T4 corresponds to the total amount of hormone produced, evaluating both the amount that is conjugated to proteins and that is freely circulating in the blood. However, the dosage of total T4 can be somewhat nonspecific, 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 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 carried out
The test is done with a blood sample and no preparation is necessary before taking it. However, if the person is taking any medication that interferes with the thyroid, they must inform the doctor so that this can be taken into account at the time of the analysis.
Reference value
The normal values of free T4 are between 0.9 – 1.8 ng/dLwhile normal total T4 values vary according to age:
Elevated or decreased T4 values may indicate hypo or hyperthyroidism, thyroid cancer, thyroiditis, goiter and female infertility, for example. Furthermore, decreased free T4 values may indicate poor nutrition or Hashimoto’s thyroiditis, for example, which is an autoimmune disease characterized by inflammation of the thyroid leading to hyperthyroidism followed by hypothyroidism.
When to do
The T4 test is normally 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. Discover other essential exams to evaluate the thyroid.
