A TSH test measures the amount of thyroid stimulating hormone (TSH) in your blood. TSH is produced by the pituitary gland. It tells the thyroid gland to make and release thyroid hormones into the blood.
Thyrotropin; Thyroid stimulating hormone; Hypothyroidism - TSH; Hyperthyroidism - TSH; Goiter - TSH
How the Test is Performed
A blood sample is needed. Other thyroid tests that may be done at the same time include:
How to Prepare for the Test
There is no preparation needed for this test. Ask your health care provider about any medicines you are taking that may affect the test results. DO NOT stop taking any medicines without first asking your provider.
Medicines you may need to stop for a short time include:
- Potassium iodide
- Prednisone or other glucocorticoid medicines
How the Test will Feel
When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or a slight bruise. This soon goes away.
Why the Test is Performed
Your provider may also check your TSH level if you are planning to get pregnant.
Normal values range from 0.4 to 4.0 milli-international units per liter (mIU/L).
- TSH values can vary during the day. It is best to have the test early in the morning.
- Experts do not fully agree on what the upper number should be. Opinions vary from 2.5 to 4.0 mIU/L.
Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your provider about the meaning of your specific test results.
If you are being treated for a thyroid disorder, your TSH level will likely be kept between 0.5 and 4.0 mIU/L, except when:
- A pituitary disorder is the cause of the thyroid problem. A low TSH may be expected.
- Someone has been treated for thyroid cancer. A low TSH may be best to prevent the thyroid cancer from coming back.
- A woman is pregnant. The normal range for TSH is different for women who are pregnant. Your provider may suggest that you take thyroid hormone, even if your TSH is in the normal range.
What Abnormal Results Mean
A higher-than-normal TSH level is most often due to an underactive thyroid gland (hypothyroidism). There are many causes of this problem.
A lower-than-normal level may be due to an overactive thyroid gland, which can be caused by:
- Graves disease
- Toxic nodular goiter or multinodular goiter
- Too much iodine in the body (due to receiving iodine contrast used during imaging tests, such as CT scan)
- Taking too much thyroid hormone medicine or prescribed natural or over-the-counter supplements that contain thyroid hormone
Use of certain medicines may also cause a lower-than-normal TSH level. These include glucocorticoids/steroids, dopamine, certain chemotherapy drugs, and opioid painkillers such as morphine.
Veins and arteries vary in size from one person to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
Other risks associated with having blood drawn are slight but may include:
- Excessive bleeding
- Fainting or feeling lightheaded
- Hematoma (blood buildup under the skin)
- Infection (a slight risk any time the skin is broken)
Salvatore D, Davies TF, Schlumberger MJ, Hay ID, Larsen PR. Thyroid physiology and diagnostic evaluation of patients with thyroid disorders. In: Melmed S, Polonsky KS, Larsen PR, Kronenberg HM, eds. Williams Textbook of Endocrinology. 13th ed. Philadelphia, PA: Elsevier; 2016:chap 11.
Weiss RE, Refetoff S. Thyroid function testing. In: Jameson JL, De Groot LJ, eds. Endocrinology: Adult and Pediatric. 7th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 78.