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

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 the next tab "The test/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?

Looking for reference ranges?

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

How is hyperthyroidism treated?

Hyperthyroidism can be controlled through treatment. This will normally involve either tablets which stop the thyroid gland producing thyroid hormones, radioiodine treatment which destroys thyroid tissue, or surgery to remove part or all of the thyroid gland.

How is hypothyroidism treated?

Hypothyroidism is easily treated and controlled for most people with thyroxine (T4) replacement in the form of a tablet.

Do doctors test for TSH levels during pregnancy?

Yes, that is a good way to test for hypothyroidism during pregnancy, which is often overlooked.

For more information, see British Thyroid Association: Underactive Thyroid and Pregnancy


Last Review Date: February 24, 2013