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
How the Test is Performed
A blood sample is needed.
Other thyroid tests that may be done at the same time include:
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 health care provider.
Medicines you may need to temporarily stop include:
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 doctor will order this test if you have symptoms or signs of an overactive or underactive thyroid gland. It is also used to monitor treatment of these conditions.
Normal values range from 0.4 - 4.0 milli-international units per liter (mIU/L). Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your doctor 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 in these situations:
For a pituitary disorder, a low TSH may be appropriate.
For thyroid cancer, a low TSH may be appropriate to prevent the thyroid cancer from coming back.
The normal range for TSH is different for women who are pregnant. Your doctor may suggest that you take thyroid hormone, even if your TSH is in the normal range.
TSH values can vary during the day. It is best to have the test early in the morning.
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:
Too much iodine in the body (due to receiving iodine contrast used during imaging tests, such as CT scan)
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 patient 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:
Fainting or feeling light-headed
Hematoma (blood accumulating under the skin)
Infection (a slight risk any time the skin is broken)
Guber HA, Farag AF. Evaluation of endocrine function. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 24.
Salvatore D, Davies TF, Schlumberger MJ, et al. Thyroid physiology and diagnostic evaluation of patients with thyroid disorders. In: Melmed S, Polonsky KS, Larsen PR, et al., eds. Williams Textbook of Endocrinology. 12th ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 11.
Brent Wisse, MD, Associate Professor of Medicine, Division of Metabolism, Endocrinology & Nutrition, University of Washington School of Medicine, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.