A TSH test is a lab test that 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. For information on how this is done, see: Venipuncture
Other 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 health care provider.
Medicines you may need to stop taking include:
- Potassium iodide
How the test will feel
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
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 can range from 0.4 - 4.0 mIU/L (milli-international units per liter), depending on:
- Your symptoms
- Results of other thyroid lab tests
- Whether you are already being treated for thyroid problems
Even without signs or symptoms of an underactive thyroid (hypothyroidism), you will need to be followed closely by your doctor if your TSH level is over 3.5 mIU/L but your T4 test is normal (called subclinical hypothyroidism).
If you are being treated for a thyroid disorder, your TSH level should be between 0.5 and 2.0 mIU/L.
The examples above are common measurements for results of these tests. 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.
What abnormal results mean
Higher-than-normal TSH levels are most often due to an underactive thyroid gland (hypothyroidism). There are many causes of this problem.
Lower-than-normal levels may be due to an overactive thyroid gland, which can be caused by:
What the risks are
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:
- Excessive bleeding
- Fainting or feeling light-headed
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
Kim M, Ladenson P. Thyroid. In: Goldman L, Schafer AI, eds. Cecil Medicine. 24th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 233.
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. 12th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 11.
Last reviewed 6/26/2012 by Shehzad Topiwala, MD, Chief Consultant Endocrinologist, Premier Medical Associates, The Villages, FL. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M. Health Solutions, Ebix, Inc.
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