Thyroid function tests are blood tests which help to check the function of your thyroid gland. They are mainly used to detect an underactive thyroid gland (hypothyroidism) and an overactive thyroid gland (hyperthyroidism).
Lots of basic information about the thyroid gland can be found in the separate leaflet called Thyroid Problems (also including Parathyroid Glands). This leaflet will only deal with the blood test that measures how your thyroid is working.
Note: the information below is a general guide only. The arrangements, and the way tests are performed, may vary between different hospitals. Someone can have their thyroid function test in one hospital and it could give slightly different results to having it measured in another hospital. It is important to interpret the results of your thyroid function test with a doctor.
What is a thyroid function test?
A thyroid function test is a blood test to check the levels of the hormones made by your thyroid gland. The test also checks the level of a hormone made by the pituitary gland in your brain, which acts on your thyroid gland.
The two hormones, thyroxine (T4) and thyroid-stimulating hormone (TSH), work together and are usually in balance. In a healthy person the brain produces just the right amount of TSH to keep the thyroid gland ticking over. The thyroid gland then produces just the right amount of thyroxine.
This is called a feedback loop: if the thyroid gland makes too much thyroxine it will feed back to the brain to make less TSH. This diagram shows the feedback loop between the thyroid gland in your neck and the pituitary gland in the brain:
Thyroid function test results explained
The thyroid usually changes slowly. It hardly ever quickly changes to being overactive or underactive: it usually takes a few weeks at least, or even a few months. If you have a thyroid function test that shows something is wrong, it's usually worth repeating it in 3-6 weeks. That is because there's always the chance that your thyroid gland could have gone back to normal by itself.
The second thing to bear in mind about thyroid function tests is that medications, and even herbal remedies or vitamin supplements, can affect the accuracy of thyroid results.
For these two reasons it's always recommended to have a thyroid function test if it is recommended to you by a doctor, rather than having it done privately or without a doctor suggesting it.
The following are approximate values for what is normal for TSH and thyroxine levels. These are a rough guide only and will vary from hospital to hospital.
- Normal range for thyroid-stimulating hormone: 0.4-4.0 mU/L.
- Normal range for thyroxine: 9-24 pmol/L.
So in general if your TSH is low and your thyroxine level is high, you may have an overactive thyroid gland.
Conversely if your TSH is high and your thyroxine level is low, you may have an underactive thyroid gland.
There are other options such as having a high TSH but a normal thyroxine and that can indicate something called subclinical hypothyroidism.
Be aware that there are other conditions that can cause abnormalities with the thyroid function tests (such as adrenal insufficiency) and so the pointers above are a guide only: you should discuss your thyroid function tests with your doctor before reaching any conclusions.
What are thyroid function tests used for?
Thyroid function tests are usually done to find out whether the thyroid gland is working properly. This is mainly to diagnose an underactive thyroid gland (hypothyroidism) and an overactive thyroid gland (hyperthyroidism).
People with some conditions have an increased risk of thyroid problems and so are often advised to have thyroid function tests undertaken each year. This conditions include:
Thyroid function tests can also be done to:
- Monitor treatment with thyroid replacement medicine for people who have hypothyroidism.
- Check thyroid gland function in people who are being treated for hyperthyroidism.
- Screen newborn babies for inherited problems with the thyroid gland.
What happens during a thyroid function test?
A thyroid function test is a simple blood test. The blood sample is then sent to the laboratory for analysis and the results are sent back to the doctor who asked for the tests. The results usually take 1-3 days to come back.
What should I do to prepare for a thyroid function test?
Thyroid function tests usually require very little preparation.
You don't need to fast before the blood test. And it doesn't matter if you have taken your thyroid medicine just before the blood test.
Tell your doctor if you are taking any medication, as some medicines can alter the test results and how they are interpreted.
It is also important to mention if you have had any X-ray tests that have used a special contrast dye, as this may contain iodine which can affect the results. Levels of thyroid chemicals (hormones) also change in pregnancy, so tell your doctor if you are pregnant when the test is taken.
Note: all newborn children have their thyroid function tested as part of the heel prick test which is offered to all babies and undertaken when they are 5 days old. See the separate leaflet called Newborn Baby Screening Tests for more information.
Further reading and references
Thyroid disease: assessment and management; NICE (November 2019)