T4 is a hormone produced by the thyroid that is responsible for promoting metabolism so that the body has sufficient energy needed to function properly. T4 is almost entirely bound to proteins so that it can be transported in the bloodstream to various organs and perform its function.
Free T4 describes the amount of T4 that is not bound to proteins and actively performing its function in the body. Total T4 describes the amount of T4 bound and not bound to proteins.
This test may be recommended by the doctor in routine exams, but it is particularly ordered to investigate for hypo or hyperthyroidism, or when the patient presents with an abnormal TSH result. Read more about the TSH blood test and what it's for.
What is it
T4 is a very important hormone for metabolism. Its levels are checked to determine whether the thyroid is producing a normal and sufficient amount of hormones to ensure the body has enough energy to carry out its metabolic activities.
Less than 1% of T4 is found in free form, which means it is not bound to a protein. This means it is it actively working and playing its role. T4 that is bound to a protein has no activity, and is only being transported in the bloodstream to the organs. When necessary, it will then separated from the protein to play its role.
Total T4 describes to the total amount of T4 hormone produced in the thyroid. This result takes into account both the amount of T4 that is bound to a protein and the amount that is circulating freely n the blood. However, total T4 levels can be somewhat nonspecific, which is why these results are not always assessed in isolation.
Free T4, on the other hand, is more specific, sensitive and allows for a better assessment of the thyroid, since only the amount of hormone that is functional and active in the body is measured.
How the test is performed
A free T4 test is performed by collecting a blood sample. No preparation is necessary for this test. However, if the person is taking any medication that interferes with the thyroid, they should inform their doctor so that this can be taken into account during the analysis.
Normal levels
Normal free T4 values are between 0.9 - 1.8 ng/dL, while normal total T4 values vary according to age:
High or low T4 values may indicate hypo or hyperthyroidism, thyroid cancer, thyroiditis, goiter and female infertility, for example. Furthermore, low free T4 levels may be a sign of malnutrition or Hashimoto's thyroiditis, which is an autoimmune disease characterized by inflammation of the thyroid leading to hyperthyroidism followed by hypothyroidism.
When it's tested
The T4 test is usually requested by the endocrinologist in situations such as:
- Abnormal TSH test results
- Weakness, decreased metabolism and fatigue, 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 identify a diagnosis, which will help to guide the most appropriate treatment approach.
Learn about other thyroid tests your doctor may order to evaluate thyroid function.