The TSH test is often the test of choice for evaluating thyroid function and/or symptoms of hyper- or hypothyroidism. It is frequently ordered along with or preceding a T4 test. Other thyroid tests that may be ordered include a T3 test and thyroid antibodies (if autoimmune-related thyroid disease is suspected).

TSH testing is used to:

diagnose a thyroid disorder in a person with symptoms,

screen newborns for an underactive thyroid,

monitor thyroid replacement therapy in people with hypothyroidism

diagnose and monitor female infertility problems,

help evaluate the function of the pituitary gland (occasionally), and

screen adults for thyroid disorders, although expert opinions vary on who can benefit from screening and at what age to begin.

The doctor may order a TSH test when a patient has symptoms of hyper- or hypothyroidism and/or when a patient has an enlarged thyroid gland. It may be ordered at regular intervals to monitor the effectiveness of treatment when a patient is being treated for a known thyroid disorder.

TSH screening is routinely performed in the United States on newborns as part of each state’s newborn screening program. The American Thyroid Association recommends that adults older than age 35 be screened for thyroid disease with a TSH test every five years, although other organizations, such as the U.S. Preventive Services Task Force, challenge this recommendation. Several organizations recommend instead screening women over 50 or those at high risk for thyroid disorders, such as pregnant and postpartum women.

A high TSH result often means an underactive thyroid gland that is not responding adequately to the stimulation of TSH due to some type of acute or chronic thyroid dysfunction. Rarely, a high TSH result can indicate a problem with the pituitary gland, such as a tumor producing unregulated levels of TSH. A high TSH value can also occur when patients with a known thyroid disorder (or those who have had their thyroid gland removed) are receiving too little thyroid hormone medication.

A low TSH result can indicate an overactive thyroid gland (hyperthyroidism) or excessive amounts of thyroid hormone medication in those who are being treated for an underactive (or removed) thyroid gland. Rarely, a low TSH result may indicate damage to the pituitary gland that prevents it from producing adequate amounts of TSH.

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.

The following table summarizes test results and their potential meaning.

TSH	T4	T3	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

Many medications — including aspirin and thyroid-hormone replacement therapy — may affect thyroid gland 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 to 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.