Thyroid-stimulating Hormone (TSH)

The thyroid-stimulating hormone (TSH) test is often the test of choice for evaluating thyroid function and/or symptoms of a thyroid disorder, including hyperthyroidism or hypothyroidism.

TSH is produced 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 and to help control the rate at which the body uses energy.

A TSH test is frequently ordered along with or preceding a free T4 test. Other thyroid tests that may be ordered include a free T3 test and thyroid antibodies (if autoimmune-related thyroid disease is suspected). Sometimes TSH, free T4 and free T3 are ordered together as a thyroid panel.

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
• Monitor anti-thyroid treatment in people with hyperthyroidism
• Help diagnose and monitor infertility problems in women
• Help evaluate the function of the pituitary gland (occasionally)
• Screen adults for thyroid disorders, although expert opinions vary on who can benefit from screening and at what age to begin

A health practitioner may order a TSH test when someone has symptoms of hyperthyroidism or hypothyroidism and/or when a person has an enlarged thyroid gland (goiter).

Signs and symptoms of hyperthyroidism may include:

• Increased heart rate
• Anxiety
• Weight loss
• Difficulty sleeping
• Tremors in the hands
• Weakness
• Diarrhea (sometimes)
• Light sensitivity, visual disturbances
• The eyes may be affected: puffiness around the eyes, dryness, irritation, and, in some cases, bulging of the eyes.

Signs and symptoms of hypothyroidism may include:

• Weight gain
• Dry skin
• Constipation
• Cold intolerance
• Puffy skin
• Hair loss
• Fatigue
• Menstrual irregularity in women

TSH may be ordered at regular intervals when an individual is being treated for a known thyroid disorder. When a person's dose of thyroid medication is adjusted, the American Thyroid Association recommends waiting 6-8 weeks before testing the level of TSH again.

TSH screening is routinely performed in the United States on newborns soon after birth as part of each state's newborn screening program.

In 2004, the U.S. Preventive Services Task Force found insufficient evidence to recommend for or against routine screening for thyroid disease in asymptomatic adults. However, the American Thyroid Association and the American Association of Clinical Endocrinologists released clinical practice guidelines in 2012 that recommend that screening for hypothyroidism should be considered in people over the age of 60. Because the signs and symptoms of both hypothyroidism and hyperthyroidism are so similar to those seen in many common disorders, health practitioners often need to rule out thyroid disease even though the patient has another problem.

A high TSH result may mean that:

• The person tested has an underactive thyroid gland that is not responding adequately to the stimulation of TSH due to some type of acute or chronic thyroid dysfunction
• A person with hypothyroidism or who has had their thyroid gland removed is receiving too little thyroid hormone replacement medication and the dose may need to be adjusted
• A person with hyperthyroidism is receiving too much anti-thyroid medication and the dose needs adjusting
• There is a problem with the pituitary gland, such as a tumor producing unregulated levels of TSH

A low TSH result may indicate:

• An overactive thyroid gland (hyperthyroidism)
• Excessive amounts of thyroid hormone medication in those who are being treated for an underactive (or removed) thyroid gland
• Insufficient anti-thyroid medication in a person being treated for hyperthyroidism; however, it may take a while for TSH production to resume after successful anti-thyroid treatment. This is why the American Thyroid Association recommends monitoring this treatment with tests for thyroid hormones (T4 and T3) as well as TSH levels.
• 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 some examples of typical test results and their potential meaning.

It is important to note that TSH, free T4, and free T3 tests are a "snapshot" of what is occurring within a dynamic system. An individual person's thyroid testing results may vary and may be affected by:

• Increases, decreases, and changes (inherited or acquired) in the proteins that bind T4 and T3
• Pregnancy
• Estrogen and other drugs
• Liver disease
• Systemic illness
• Resistance to thyroid hormones

Many medications — including aspirin and thyroid-hormone replacement therapy — may affect thyroid gland function test results and their use should be discussed with the health practitioner prior to testing.

Illnesses not directly related to the thyroid, "nonthyroidal illnesses," can affect thyroid hormones levels. In particular, the level of T3 can be low in nonthyroidal illness (NTI). Typically, the thyroid hormone levels return to normal after a person recovers from the nonthyroidal illness. Historically, this condition was referred to as "euthyroid sick syndrome" but that term is controversial because there is some question as to whether those affected have a thyroid gland that is functioning normally (euthyroid).

When a health practitioner adjusts a person's thyroid hormone replacement dosage, it is important to wait at least one to two months before checking the TSH again so that the new dose can have its full effect.

Extreme stress and acute illness may affect TSH test results. It is generally recommended that thyroid testing be avoided in hospitalized patients or deferred until after a person has recovered from an acute illness.

Results may be low during the first trimester of pregnancy.

Health practitioners do not generally test asymptomatic women, but those with symptoms and/or a known thyroid disorder may be tested at intervals to detect and evaluate hyperthyroidism or hypothyroidism both during pregnancy and after. For more information, see the National Endocrine and Metabolic Diseases Information Service webpage: Pregnancy and Thyroid Disease.