Thyroid-Stimulating Hormone (TSH)

The TSH test is often the first test for evaluating thyroid function and/or symptoms of hyper- or hypothyroidism. It is frequently measured together with thyroxine (usually Free T4). Other thyroid tests that may be requested include T3 (usually as Free T3) and thyroid antibodies (if autoimmune-related thyroid disease is suspected). 

TSH testing is used to: 

• screen newborns for an underactive thyroid 
• diagnose a thyroid disorder in a person with symptoms of thyroid disease 
• monitor thyroid replacement therapy in patients with hypothyroidism 
• monitor the response to treatment in patients with hyperthyroidism 
• as part of the investigation of female infertility problems 
• to help evaluate the function of the pituitary gland 
• monitor thyroxine replacement in thyroid cancer patients after removal of thyroid tissue

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 test may also be requested to monitor the effectiveness of treatment when a patient is being treated for a known thyroid disorder. 

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 the UK. 

A high TSH result often means an underactive thyroid gland caused by failure of the gland (hypothyroidism) although small elevations of TSH can be transitory due to non-thyroidal illness. Very rarely, a high TSH result can indicate a problem with the pituitary gland, such as a tumour, producing uncontrolled concentrations of TSH or when there is resistance to the action of thyroid hormones. A high TSH value can also occur in people with an underactive thyroid gland 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.  

Whether high or low, an abnormal TSH indicates an excess or deficiency in the amount of thyroid hormone available to the body, but it does not indicate the reason why. An abnormal TSH test result is usually followed by additional testing (of FT4 and sometimes FT3) to investigate the cause of the increase or decrease. 

Thyroid-hormone replacement therapy and some drugs may interfere with thyroid function test results, so tell your doctor about any drugs you are taking, for example, lithium and amiodarone. 

When your doctor adjusts your dose of thyroid hormone, it is important that you wait at least 6 weeks 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. This is known as non-thyroidal illness. TSH concentration may be low during the first trimester of pregnancy. 

There is currently no recommendation for screening of patients that are not symptomatic of thyroid disease. The frequency of testing of patients with a known abnormality of TSH is dependent on whether treatment is required and the risk of the patient needing treatment in the future. Mild increases in TSH in a a patient without symptoms, may be monitored annually or up to every 3 years depending on the risk of the patient progressing to thyroid disease.  

In patients treated for an underactive thyroid (hypothyroidism), TSH should be monitored every 3 months until the TSH has stabilised. Once the TSH has stabilised then annual testing should be performed.  

In patients with a diagnosis of an overactive thyroid (hyperthyroidism) more regular TSH monitoring may be required (up to every 6 weeks during early stages of treatment).  

Measurement of TSH should not routinely be performed more frequently than every 6 weeks to allow sufficient time for TSH concentrations to stabilise after a change in treatment.