What is being tested?
Thyroid function tests are used to check for a number of thyroid problems. The tests measure the amount of different hormones including thyroid stimulating hormone (TSH), thyroxine (T4), and triiodothyronine (T3). Other tests on the thyroid include various antibodies related to thyroid tissue.
TSH is produced by the pituitary gland in the brain. TSH is carried in the blood to the thyroid gland, which sits at the front of the neck. From there, TSH influences how much T4 and T3 are released into the blood.
TSH, T3 and T4 levels can all be tested in the blood. Your doctor will probably test TSH first, then T3 and T4 if TSH is abnormal. Tests can also be used to measure antibodies to diagnose autoimmune thyroid disease.
Why would I need these tests?
The thyroid gland's hormones help control some of your body's metabolic processes, such as heart rate, blood pressure, body temperature and weight. Too much or too little of these hormones can make you ill.
You might need thyroid tests if:
- you have symptoms of hyperthyroidism (overactive thyroid) or hypothyroidism (underactive thyroid)
- you are taking some form of thyroid hormone replacement treatment
- you are female and being investigated for infertility
Very rarely, babies are born without a working thyroid gland. For this reason, all Australian newborns are screened for hypothyroidism with a TSH test using a drop of blood taken from a heel prick.
How to prepare for this test
No preparation is usually needed for this test. In many cases, it can be done at any time of the day without fasting, but do check with your doctor ahead of time. Also let your doctor know if you are taking any medications before the test.
Understanding your results
A high TSH result might mean:
- that you are receiving too little thyroid hormone medication to treat hypothyroidism
- there might be a problem with your pituitary gland
A low TSH result might mean:
- you are taking too much thyroid hormone medication to treat hypothyroidism
Your doctor will be aware of your circumstances and can discuss what the results mean for you.
About thyroid function testing
About blood testing
Visit our 'Guide to blood testing' to learn more about blood tests in general with information such as:
- what to consider before having the test
- what happens during a blood test
- results accuracy
- blood tests cost
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Last reviewed: August 2020