The T4 exam aims to assess the functioning of the thyroid by measuring the total T4 and free T4 hormone. Under normal conditions, the hormone TSH stimulates the thyroid to produce T3 and T4, which are hormones responsible for assisting the metabolism, providing the necessary energy for the correct functioning of the body. T4 is almost completely conjugated to proteins so that it can be transported in the bloodstream to various organs and can perform its function.
This test may be recommended by the doctor in routine examinations, but it is best indicated when the person has symptoms of hypo or hyperthyroidism, for example, or when there is an altered TSH result. See what the TSH test and reference values are for.
What is total T4 and free T4?
Both free T4 and total T4 are used to assess thyroid function, that is, to verify whether the gland 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, it is separated from the protein for activity.
The total T4 corresponds to the total amount of hormone produced, being evaluated both the amount that is conjugated to proteins and that which is free circulating in the blood. However, the total T4 dosage may be a little nonspecific, as there may be interference with the proteins that the hormone can bind to.
Free T4, on the other hand, is already more specific, sensitive and allows a better assessment of the thyroid, since only the amount of hormone that is functional and active in the body is measured
How the exam is done
The test is done with a blood sample and no preparation is necessary before taking it. However, if the person is using any medication that interferes with the thyroid, he must inform the doctor so that this is taken into account when analyzing.
The collected blood sample is sent to the laboratory, where the free and total T4 dosage is made. Normal values for free T4 are between 0.9 - 1.8 ng / dL, while normal values for total T4 vary according to age:
| Age | Normal values of total T4 |
| 1st week of life | 15 µg / dL |
| Until 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 | 7.3 - 15 µg / dL |
| Between 5 and 12 years | 6.4 - 13.3 µg / dL |
| From 12 years old | 4.5 - 12.6 µg / dL |
Elevated or decreased T4 values may indicate hypo or hyperthyroidism, thyroid cancer, thyroiditis, goiter and female infertility, for example. In addition, decreased values of free T4 may indicate malnutrition 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 exam 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 female infertility.
From the assessment 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. Learn about other essential tests to evaluate your thyroid.
