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,

monitor thyroid replacement therapy in people with hypothyroidism, 

as part of the investigation of female infertility problems

to help evaluate the function of the pituitary gland.

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). Very rarely, a high TSH result can indicate a problem with the pituitary gland, such as a tumour, producing uncontrolled levels 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.

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.