For screening: All newborns are screened for congenital hypothyroidism. There is currently no recommendation in the UK for routine screening of adults
For monitoring treatment: as directed by your doctor
Otherwise: when a patient has symptoms of hypo- or hyperthyroidism and/or an enlarged thyroid
A blood sample taken from a vein in the arm. For neonatal screening blood is collected by pricking the heel
The test measures the amount of thyroid-stimulating hormone (TSH) in your blood, which is an indicator of thyroid disease. TSH is made by the pituitary gland, a tiny organ located below the brain and behind the sinus cavities. It is part of the body’s feedback system to maintain stable amounts of the thyroid hormones thyroxine (T4) and triiodothyronine (T3) in the blood.
Thyroid hormones help control the rate at which the body uses energy. When concentrations decrease in the blood, the hypothalamus (an organ in the brain) releases thyrotropin releasing hormone (TRH). This stimulates the release of TSH by the pituitary gland. The TSH in turn stimulates the production and release of T4 and T3 by the thyroid gland, a small butterfly-shaped gland that lies in the neck flat against the windpipe. When all three organs are functioning normally, thyroid hormone concentrations in the blood remain constant.
How is the sample collected for testing?
A blood sample obtained from a needle placed in a vein in your arm.
Is any test preparation needed to ensure the quality of the sample?
How is it used?
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:
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 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.
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, producing uncontrolled concentrations of TSH. 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.
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 to investigate the cause of the increase or decrease.
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.
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?
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: For patients with underactive thyroid (hypothyroidism) who are pregnant or hoping to become pregnant