Thyroid-stimulating Hormone (TSH)
A blood sample drawn from a vein in your arm or from pricking the heel of an infant
No test preparation is needed. However, certain medications, multivitamins and supplements can interfere with the TSH test, so tell your healthcare practitioner about any prescribed or over-the-counter drugs and/or supplements that you are taking. If you take thyroid hormone as treatment for thyroid disease, it is recommended that your blood sample be drawn before you take your dose for that day. 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.
Thyroid-stimulating hormone (TSH) is produced by the anterior pituitary gland, a small organ located below the brain and behind the sinus cavities. TSH stimulates the thyroid, a small butterfly-shaped gland located inside the neck in front of the windpipe, by binding to the TSH receptor to release the hormones thyroxine (T4) and triiodothyronine (T3) into the blood. This test measures the amount of TSH in the blood.
T4 and T3 help control the rate at which the body uses energy. Most of the hormone produced by the thyroid is T4. This hormone is relatively inactive, but it is converted into the much more active T3 in the liver and other tissues.
TSH, along with its regulatory hormone thyrotropin releasing hormone (TRH), which comes from the hypothalamus, is part of the feedback system that the body uses to maintain stable amounts of thyroid hormones in the blood.
- When thyroid hormone levels decrease in the blood, the pituitary gland produces more TSH in response to TRH stimulation. TSH in turn stimulates the thyroid to produce and release more T4 and T3.
- When thyroid hormone levels increase in the blood, the pituitary gland produces less TSH, and the thyroid produces less T4 and T3.
When all three organs (hypothalamus, pituitary and thyroid) are functioning normally, thyroid production is regulated to maintain relatively stable levels of thyroid hormones in the blood.
If the thyroid releases inappropriately large amounts of T4 and T3, the affected person may experience symptoms associated with overactive thyroid (hyperthyroidism), such as rapid heart rate, weight loss, nervousness, hand tremors, irritated eyes, and difficulty sleeping. Graves disease is the most common cause of hyperthyroidism. It is a chronic autoimmune disorder in which the affected person's immune system produces autoantibodies that act like TSH, bind and activate the TSH receptor, leading to the production of excessive amounts of thyroid hormone. In response, the pituitary produces less TSH, usually leading to a low level in the blood.
If there is decreased production of thyroid hormones by the thyroid (underactive thyroid or hypothyroidism), the person may experience symptoms such as weight gain, dry skin, constipation, cold intolerance, and fatigue. Hashimoto thyroiditis is the most common cause of hypothyroidism in the U.S. It is a chronic autoimmune condition in which the immune response causes inflammation and damage to the thyroid as well as the production of autoantibodies. However, the autoantibodies do not cause the hypothyroidism. The detection of thyroid-related autoantibodies (e.g., thyroperoxidase autoantibodies and/or thyroglobulin autoantibodies) indicate that thyroid autoimmunity is present. These autoantibodies can be detected in Graves disease or Hashimoto thyroiditis. With Hashimoto thyroiditis, the thyroid produces low levels of thyroid hormone. In response, the pituitary normally produces more TSH, usually resulting in a high level in the blood.
However, the level of TSH alone does not always predict or reflect thyroid hormone levels. Some people with pituitary disease produce an abnormal form of TSH that does not function properly. They often have hypothyroidism despite having normal or even mildly elevated TSH levels.
Rarely, pituitary dysfunction may result in increased or decreased amounts of TSH. In addition to pituitary dysfunction, hyperthyroidism or hypothyroidism can occur if there is a problem with the hypothalamus (insufficient or excessive TRH).
How is the test used?
The thyroid-stimulating hormone (TSH) test is often the test of choice for initially evaluating thyroid function and/or symptoms of a thyroid disorder, including overactive or underactive thyroid (hyperthyroidism or hypothyroidism).
A TSH test is frequently ordered along with or prior to a free T4 test. Other thyroid tests that may be ordered include total or free T3 tests and thyroid antibodies (if autoimmune-related thyroid disease is suspected). TSH, free T4 and sometimes free T3 may be ordered together as a thyroid panel. Total T3 and free T3 should not both be ordered.
TSH testing may be used with free T4 and sometimes total or free T3 tests to:
- Help diagnose a thyroid disorder in a person with signs and symptoms
- Monitor thyroid replacement therapy in people with hypothyroidism
- Monitor anti-thyroid treatment in people with hyperthyroidism
- Help evaluate the function of the pituitary gland (occasionally)
At present, screening the general population for thyroid disorders is not recommended. However, expert opinions vary on this. In 2015, the U.S. Preventive Services Task Force found insufficient evidence to recommend for or against routine screening for thyroid disease in asymptomatic adults.
On the other hand, guidelines released in 2012 by the American Thyroid Association and the American Association of Clinical Endocrinologists summarize recommendations endorsed by several societies. They say 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, healthcare practitioners often need to rule out thyroid disease even though the patient has another problem.
When is it ordered?
Signs and symptoms of hyperthyroidism may include:
- Increased heart rate
- Weight loss
- Difficulty sleeping
- Tremors in the hands
- Diarrhea (sometimes)
- Light sensitivity, visual disturbances
- There may be puffiness around the eyes with dryness, irritation, and, in some cases, bulging of the eyes.
- Menstrual irregularity in women
Signs and symptoms of hypothyroidism may include:
- Weight gain
- Dry skin
- Cold intolerance
- Puffy skin
- Hair loss
- Menstrual irregularity in women and/or fertility problems 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.
What does the test result mean?
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; Hashimoto thyroiditis is the most common cause of hypothyroidism.
- 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 rare inherited disorder is present in which the body and/or pituitary do not respond normally to thyroid hormones, resulting in high TSH despite clinically normal thyroid function
A low TSH result may indicate:
- An overactive thyroid gland (hyperthyroidism); Graves disease is the most common cause of hyperthyroidism.
- Excessive amounts of thyroid hormone medication taken by 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 (free T4 and total and free T3) as well as TSH levels.
- Damage to the pituitary gland that prevents it from producing adequate amounts of TSH
- People with thyroid cancer may be treated with medications intended to suppress thyroid hormones, so they may have a low 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.
Note: Laboratory results must always be correlated with the clinical findings of the patient. TSH Free T4 Total or Free T3 Most likely diagnosis Normal Normal Normal Normal thyroid function (e.g., "euthyroid") Normal or decreased Normal or decreased Decreased Normal adjustment in thyroid function due to illness (nonthyroidal illness or sick euthyroid syndrome) Increased Normal Normal Subclinical hypothyroidism1; in a person with hypothyroidism on treatment, not enough thyroid hormone is being given Increased Decreased Normal of decreased Hypothyroidism resulting from a problem with the thyroid gland itself (primary hypothyroidism) Normal or increased Increased Increased Hyperthyroidism resulting from a problem with the pituitary gland signals (central hyperthyroidism) or from a problem with the thyroid hormone receptor (thyroid hormone resistance) Decreased Normal Normal Subclinical hyperthyroidism2; in a person with hypothyroidism, too much thyroid hormone is being given Decreased Normal Increased Hyperthyroidism resulting from the thyroid gland making too much active thyroid hormone T3 (uncommon, also known as T3 toxicosis) Decreased Increased Increased Hyperthyroidism resulting from the gland making too much thyroid hormones (primary hyperthyroidism) Decreased Decreased Decreased Hypothyroidism resulting from a problem with the hypothalamus or pituitary signals that govern the thyroid gland (central hypothyroidism)
1In affected adults, the diagnosis of subclinical hypothyroidism is applied when the TSH level is elevated and the free T4 level is normal on repeat testing over a number of weeks or months. Adults with subclinical hypothyroidism may have few or no overt symptoms of hypothyroidism. However, subclinical hypothyroidism places affected adults at somewhat increased risk for an elevated LDL cholesterol level, increased risk for cardiovascular disease, and reduced mental acuity.
2In affected adults, the diagnosis of subclinical hyperthyroidism is applied when the TSH level is decreased and the free T4 level and T3 levels are normal on repeat testing over a number of weeks or months. Adults with subclinical hyperthyroidism may have few or no overt symptoms of hyperthyroidism. However, subclinical hyperthyroidism places affected persons at somewhat increased risk for atrial fibrillation and osteoporosis.
Is there anything else I should know?
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 thyroid hormone (This is important for tests that measure total T3 but it is not likely important for free T4 and free T3 tests, which measure the thyroid hormones that are not bound to protein.)
- Liver disease
- Systemic illness
- Rarely, resistance to thyroid hormones
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). Nevertheless, there is little data to support the treatment of NTI with thyroid hormone.
What medications can affect a TSH test?
Many multivitamins, supplements (especially hair and nail), and over-the-counter and prescription medications may affect thyroid test results and their use should be discussed with your healthcare practitioner prior to testing. For example, biotin (vitamin B7) can interfere with some lab tests, so your healthcare practitioner may advise you to refrain from taking biotin or supplements that contain biotin for a few days before having blood drawn for a TSH test. If you have a procedure done in which fluorescein dyes are injected into your blood (e.g., angiography), you may need to wait a few days before having a TSH test done.
Do healthcare practitioners test TSH during pregnancy?
Pregnancy causes normal changes in the function of many endocrine glands, including the thyroid gland. Healthcare practitioners do not generally test asymptomatic women, but those with symptoms and/or a known thyroid disorder will usually be tested at intervals to detect and monitor hyperthyroidism or hypothyroidism during pregnancy and after delivery of the baby.
Some experts have advocated screening pregnant women for elevated TSH during the first trimester (or preconception) even if they do not have a history of thyroid disease. However, most guidelines do not support this course of action.
For more information, see the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) webpage: Thyroid Disease and Pregnancy.
My newborn had a TSH test done. Why?
A TSH test may be used to screen for congenital hypothyroidism. Screening for this condition is routinely performed in the United States on newborns soon after birth as part of each state's newborn screening program. Congenital hypothyroidism occurs when a baby is born with an underactive thyroid gland or a thyroid gland that is not located where it should be or is missing completely. For more details, see the information at the Hormone Health Network's webpage on Congenital Hypothyroidism.
Are there things that I can do to raise or lower my TSH level?
What is a 3rd generation TSH and an ultrasensitive TSH?
Almost all laboratories currently use "third generation" or "ultrasensitive" TSH assays today.
The original tests (immunoassays) for TSH were not sensitive enough to differentiate the very low levels seen in patients with hyperthyroidism from levels seen in normal euthyroid individuals. In the 1980s, more sensitive assays ("second generation") were developed and these were able to identify patients with TSH levels that were suppressed due to the excess amounts of free T4 present in hyperthyroidism. In the 1990s, TSH assays were made even more sensitive and, although these were able to measure even lower levels, they were widely adopted because they performed much better than the second-generation assays in the range that was important for differentiating normal from hyperthyroid.