How is it used?
Thyroid stimualting hormone (TSH) testing is used to:
- screen newborns for an underactive thyroid gland
- diagnose a thyroid disorder in a person with
- monitor thyroid replacement therapy in people with
- 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 include heat intolerance, weight loss, rapid heartbeat, nervousness, sweating and increased frequency of stools.
Common symptoms of 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 test result mean?
A high TSH result often means an underactive thyroid gland caused by failure of the gland (). Very rarely, a high TSH result can indicate a problem with the pituitary gland, such as a , 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 () 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 |
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. 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.