Thyroid conditions are common and can affect people of all ages from babies to the elderly. They can be caused by a range of underlying conditions. Your doctor will usually start by requesting a TSH test. This measures the amount of Thyroid Stimulating Hormone (TSH) in your blood. TSH is the hormone that stimulates your thyroid gland to make thyroid hormone. This is the only thyroid test covered by Medicare if you do not have a history of thyroid problems. Depending on your symptoms and your TSH result, you may need further testing of your thyroid hormones with measurement of FreeT4 and Free T3. If your doctor thinks you could have an autoimmune condition that is affecting your thyroid gland you may go on to have thyroid antibody tests. There are also tests that are used to monitor thyroid cancer following treatment. They include thyroglobulin and calcitonin tests which are used to detect and monitor some rare forms of thyroid cancer.
What is being tested?
The test measures the amount of thyroid-stimulating hormone (TSH) in your blood. TSH is made by the pituitary gland, a small organ located just below the brain. TSH is one of the pituitary gland’s messengers — it tells the thyroid gland to start making thyroid hormone. There is a feedback system between the pituitary gland and the thyroid gland. If too much thyroid hormone (tri-iodothyronine or T3 and thyroxine or T4) is being produced by the thyroid this sends a signal to the pituitary gland which turns down the production of TSH. Conversely, if too little T3 and T4 are being produced the pituitary gland steps up production of TSH. In healthy people this system regulates itself perfectly. However, in thyroid or pituitary diseases the system becomes unbalanced. The pattern of changes can help diagnose the underlying problem.
How is it used?
Thyroid stimulating hormone (TSH) testing is used to:
When is it requested?
Your doctor requests this test if you show symptoms of a thyroid disorder. For example, symptoms of hyperthyroidism include heat intolerance, weight loss, rapid heartbeat, nervousness, sweating and increased frequency of stools.
Common symptoms of hypothyroidism include fatigue, weakness, weight gain, muscle aches, constipation, slow heart rate and cold intolerance.
The blood test is often requested with other thyroid hormone tests and after a physical examination of your thyroid. TSH screening is routinely performed in newborns. There are currently no recommendations for routine screening of adults in Australia.
What does the result mean?
A high TSH result often means an underactive thyroid gland caused by failure of the gland (hypothyroidism). Very rarely, a high TSH result can indicate a problem with the pituitary gland, such as a tumour, in what is known as secondary hyperthyroidism. A high TSH value can also occur in people with underactive thyroid glands who have been receiving too little thyroid hormone medication.
A low TSH result can indicate an overactive thyroid gland (hyperthyroidism) or damage to the pituitary gland that prevents it from producing TSH. A low TSH result can also occur in people with an underactive thyroid gland who are receiving too much thyroid hormone medication.
An abnormal TSH usually indicates a deficiency or an excess of thyroid hormones available to the body but it does not indicate the reason why. An abnormal TSH result is usually followed by additional testing of free thyroxine (FT4) and/or free tri-iodothyronine (FT3) to investigate the cause. The combination of a TSH test and tests for the thyroid hormones T3 and T4 is known as a thyroid function test (TFT).
The following table summarises patterns of thyroid function test results and their most common causes.
TSH | FT4 | FT3 | Interpretation |
Normal | Normal | Normal | Normal thyroid function |
High | Normal | Normal | Mild (sub-clinical) underactive thyroid (hypothyroidism) or acute illness |
High | Low | Low or normal | Underactive thyroid - too little thyroid hormone being produced resulting from a problem with the thyroid (hypothyroidism) |
Low | Normal | Normal | Mild (sub-clinical) overactive thyroid (hyperthyroidism). |
Low | High or normal | High or normal | Overactive thyroid - too much thyroid hormone being produced (hyperthyroidism) |
Low | Low | Low | Hypothyroidism resulting from a problem with pituitary or hypothalamus signalling that control the thyroid gland. |
Is there anything else I should know?
Thyroid-hormone replacement therapy and some drugs may interfere with thyroid function test results, so tell your doctor about any drugs you are taking.
When your doctor adjusts your dose of thyroid hormone, it is important that you wait at least one to two months before checking your TSH again, so that your new dose can have its full effect.
Extreme stress and acute illness may also affect TSH test results. In this event, TSH should be rechecked a few weeks after the illness has resolved.
Results for TSH may be low during the first trimester of pregnancy. This is because one of the hormones of pregnancy (HCG) has a TSH-like effect so the pituitary is not required to produce as much TSH. TSH results in pregnancy should be interpreted in relation to reference intervals specific for pregnancy.
Common questions
Hyperthyroidism can be controlled through treatment. This will normally involve tablets to stop the thyroid gland producing thyroid hormones, radioactive iodine treatment to destroy thyroid tissue, or surgery to remove part or all of the thyroid gland.
Hypothyroidism is easily treated and controlled for most people with thyroxine (T4) replacement in the form of a tablet.
Yes, that is a good way to test for hypothyroidism during pregnancy, which is often overlooked.
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