A thyroid stimulating hormone (TSH) test measures the amount of TSH in the blood. A blood test can measure TSH and help a doctor diagnose a thyroid condition such as hypothyroidism or hyperthyroidism.

A thyroid-stimulating hormone (TSH) test measures the amount of TSH in the blood. TSH is produced by the pituitary gland, which is located at the base of your brain. It’s responsible for regulating the amount of hormones released by the thyroid.

The thyroid is small, butterfly-shaped gland located in the front of the neck. It’s an important gland that creates three primary hormones:

  • triiodothyronine (T3)
  • thyroxine (T4)
  • calcitonin

The thyroid controls numerous different bodily functions, including metabolism and growth, through the release of these three hormones.

Your thyroid will produce more hormones if your pituitary gland produces more TSH. In this way, the two glands work together to make sure the right amount of thyroid hormones is produced.

However, when this system is disrupted, your thyroid can produce either too many or too few hormones.

A TSH test is often performed to determine the underlying cause of abnormal thyroid hormone levels. It’s also used to screen for an underactive or overactive thyroid gland. By measuring the level of TSH in the blood, your doctor can determine how well the thyroid is working.

Your doctor may order a TSH test if you’re experiencing symptoms of a thyroid disorder. Thyroid diseases can be categorized as either hypothyroidism or hyperthyroidism.


Hypothyroidism is a condition in which the thyroid produces too few hormones, causing metabolism to slow down. The symptoms of hypothyroidism include fatigue, weakness, and difficulty concentrating.

The following are some of the most common causes of hypothyroidism:

  • Hashimoto’s thyroiditis. This is an autoimmune condition that causes the body to attack its own thyroid cells. As a result, the thyroid is unable to produce a sufficient amount of hormones. The condition doesn’t always cause symptoms, so it can progress over several years before it causes noticeable damage.
  • Thyroiditis. This is an inflammation of the thyroid gland. It’s often caused by a viral infection or an autoimmune disorder, such as Hashimoto’s thyroiditis. This condition interferes with thyroid hormone production and eventually leads to hypothyroidism.
  • Postpartum thyroiditis. This is a temporary form of thyroiditis that may develop in some people after childbirth.
  • Iodine deficiency. The thyroid uses iodine to produce hormones. An iodine deficiency can lead to hypothyroidism. It’s extremely rare in the United States due to the widespread use of iodized salt. However, it’s more common in other regions of the world.


Hyperthyroidism is a condition in which the thyroid produces too many hormones, causing the metabolism to speed up. Symptoms of hyperthyroidism include increased appetite, anxiety, and difficulty sleeping.

The following are some of the most common causes of hyperthyroidism:

  • Graves’ disease. This is a common disorder in which the thyroid becomes larger and produces an excessive amount of hormones. The condition shares many of the same symptoms as hyperthyroidism and often contributes to the development of hyperthyroidism.
  • Thyroiditis. It eventually leads to hypothyroidism, but in the short term, it can also trigger hyperthyroidism. This may occur when the inflammation causes the thyroid to produce too many hormones and release them all at once.
  • Excess iodine. Having too much iodine in the body can cause the thyroid to become overactive. This typically occurs as a result of continuously using medications that contain iodine. These medicines include some cough syrups as well as amiodarone, which is used to treat heart arrhythmias.
  • Thyroid nodules. These are benign lumps that sometimes form on the thyroid. When these lumps begin to increase in size, they may become overactive and the thyroid may start producing too many hormones.

The TSH test doesn’t require any special preparation. However, it’s important to tell your doctor if you’re taking medications that might interfere with the accuracy of the TSH measurement. Some medications that could interfere with a TSH test are:

  • amiodarone
  • dopamine
  • lithium
  • prednisone
  • potassium iodide
  • biotin

You may need to avoid using these drugs before the test. However, don’t stop taking your medications unless your doctor tells you to do so.

A TSH test involves taking a sample of blood. The blood is typically drawn from a vein that’s inside the inner elbow.

A healthcare provider will perform the following procedure:

  1. First, they’ll clean the area with an antiseptic or other sterilizing solution.
  2. They’ll then tie an elastic band around your arm to make the veins swell with blood.
  3. Once they find a vein, they’ll insert a needle into the vein to draw blood. The blood will be collected in a small tube or vial attached to the needle.
  4. After they draw enough blood, they’ll remove the needle and cover the puncture site with a bandage to stop any bleeding.

The entire procedure should only take a few minutes to complete. The blood sample will be sent to a lab for analysis.

Once your doctor receives the test results, they’ll schedule an appointment with you to discuss the results and explain what they may mean.

The normal range of TSH levels is 0.4 to 4.0 milli-international units per liter. If you’re already being treated for a thyroid disorder, the normal range is 0.5 to 3.0 milli-international units per liter.

A value above the normal range usually indicates that the thyroid is underactive. This indicates hypothyroidism. When the thyroid isn’t producing enough hormones, the pituitary gland releases more TSH to try to stimulate it.

A value below the normal range means that the thyroid is overactive. This indicates hyperthyroidism. When the thyroid is producing too many hormones, the pituitary gland releases less TSH.

Depending on the results, your doctor may want to perform additional tests to confirm the diagnosis.