Free T4 is a blood test that measures the unbound and biologically available part of the thyroid hormone thyroxine. T4 is the hormone that the thyroid gland produces the most and is of great importance for the body's metabolism, energy level, body temperature, heart rate and several other important functions.
Some of the body's T4 is converted in the tissues to T3, which is the more active form of thyroid hormone. Analysis of free T4 therefore provides important information about the thyroid's hormone production and is often used together with TSH in the investigation and follow-up of thyroid disorders.
What does a T4 test show?
The test measures the concentration of free T4 in the blood. A low value can be seen when the thyroid gland produces too little hormone, for example in hypothyroidism. An elevated value can instead occur when there is overproduction of thyroid hormone, for example in hyperthyroidism.
Free T4 can also be used to monitor the effect of treatment in an already diagnosed thyroid disease. However, the result always needs to be assessed together with TSH, since the combination of the tests gives a clearer picture of how the thyroid gland and its hormonal regulation work.
When might it be relevant to measure free T4?
A T4 test may be relevant if symptoms or previous test results raise suspicion of a thyroid disorder. This may be the case, for example, in the following cases:
- pronounced fatigue or low energy level
- feeling cold or more sensitive to cold
- unexplained weight gain or weight loss
- palpitations or altered pulse
- sweating or more sensitive to heat
- dry skin, hair loss or brittle nails
- depressed mood, anxiety or difficulty concentrating
- irregular menstruation or fertility problems
- abnormal TSH levels
- follow-up of treatment for hypo- or hyperthyroidism
How is free T4 interpreted?
The result is assessed in relation to the laboratory's reference range and needs to be interpreted together with symptoms, medical history and other thyroid tests. Above all, the combination of TSH and free T4 is important for distinguishing between different types of thyroid disorders.
A low free T4 in combination with an elevated TSH may indicate primary hypothyroidism. A high free T4 together with a low TSH may instead indicate primary hyperthyroidism.
If both TSH and free T4 are low, it may in some cases be due to a disorder in the pituitary gland or hypothalamus. Deviating or contradictory results can also be affected by medication, other illness, pregnancy or analytical disorders and may therefore need to be investigated further.
Free T4 together with TSH and free T3
TSH is the hormone that stimulates the thyroid gland to produce T4 and T3. TSH is usually the first test to be tested when a thyroid disorder is suspected, while free T4 helps to assess how much available thyroid hormone is in the blood.
Free T3 can be used as a supplementary test in some situations, especially when hyperthyroidism is suspected or when TSH and free T4 do not provide a clear enough picture.
Are additional tests needed?
If an autoimmune thyroid disease is suspected, the investigation may need to be supplemented with antibody tests. TPO antibodies may be relevant when Hashimoto's disease or other autoimmune hypothyroidism is suspected, while TRAK can be tested when Graves' disease is suspected.
A T4 test can provide important information about the thyroid's hormone production, but the result alone cannot establish a diagnosis. Deviating values may need to be followed up with further testing and medical assessment.
