At a Glance
Why Get Tested?
To screen for and diagnose thyroid disorders; to monitor treatment of hypothyroidism and hyperthyroidism
When to Get Tested?
For screening: newborns; there is currently no recommendation in Australia for routine screening of adults. For monitoring treatment: as directed by your doctor. Otherwise, as symptoms present.
Sample Required?
A blood sample drawn from a vein in the arm or from pricking the heel of an infant
Test Preparation Needed?
None
The Test Sample
What is being tested?
The test measures the amount of thyroid-stimulating hormone (TSH) in your blood. TSH is made by the pituitary gland located in your brain. TSH is the pituitary gland’s messenger — 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 T3 and T4 are being produced by the thyroid then this causes negative feedback on the pituitary gland and less TSH is produced. If too little T3 and T4 are being produced then TSH production increases. In healthy people this system regulates itself perfectly. However in thyroid or pituitary diseases the system gets unbalanced and the resulting changes can help diagnose the problem. See the table in What does the test result mean?
How is the sample collected for testing?
A blood sample is obtained from a needle placed in a vein in your arm or from pricking the heel of an infant.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
The Test
How is it used?
TSH testing is used to:
- screen newborns for an underactive thyroid
- diagnose a thyroid disorder in a person with symptoms
- monitor thyroid replacement therapy in people with hypothyroidism
- as part of the investigation of female infertility problems
- help to evaluate pituitary function (occasionally)
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, insomnia and breathlessness.
Common symptoms of hypothyroidism include fatigue, weakness, weight gain, slow heart rate and cold intolerance.
The blood test may be 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 test 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 FT4 and/or FT3 to investigate the cause.
The following table summarises the results and their most common causes.
| TSH | FT4 | FT3 | Interpretation |
|---|---|---|---|
| High | Normal | Normal | Mild (subclinical) hypothyroidism |
| High | Low | Low or normal | Hypothyroidism |
| Low | Normal | Normal | Mild (subclinical) hyperthyroidism |
| Low | High or normal | High or normal |
Hyperthyroidism |
| Low | Low or normal | Low or normal |
Nonthyroidal illness; rare pituitary (secondary) hypothyroidism |
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, and results may be low during the first trimester of pregnancy.
Common Questions
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Article Sources
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