TSH is a laboratory test that measures the amount of thyroid stimulating hormone (TSH) in your blood. TSH is produced by the pituitary gland and tells the thyroid gland to make and release the hormones thyroxine (T4) and triiodothyronine (T3).

See also:

• T3 test
• T4 test

Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

Your health care provider will instruct you, if necessary, to discontinue drugs that may interfere with the test. Drugs that can affect TSH measurements include antithyroid medications, lithium, potassium iodide, amiodarone, dopamine, and prednisone.

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.

Your doctor will order this test if you have signs of abnormal thyroid function (hyperthyrodism or hypothyrodism). It is also used to monitor treatment of these conditions.

Normal values are from 0.4 to 4.0 mIU/L for those with no symptoms of abnormal thyroid function. However, those without signs or symptoms of an underactive thyroid who have a TSH value over 2.0 mIU/L but normal T4 levels may develop hypothyroidism sometime in the future. This is called subclinical hypothyroidism (mildly underactive thyroid) or early-stage hypothyroidism. Anyone with a TSH value above this level should be followed very closely by a doctor.

If you are being treated for a thyroid disorder, your TSH level should be between 0.5 and 2.0 mIU/L.

Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

Greater-than-normal levels may indicate:

• Congenital hypothyroidism (cretinism)
• Exposure to mice (lab workers or veterinarians)
• Primary hypothyroidism
• Thyroid hormone resistance
• TSH-dependent hyperthyroidism

Lower-than-normal levels may be due to:

• Hyperthyroidism
• TSH deficiency
• Use of certain medications including dopamine agonists, glucocorticoids, somatostatin analogues, and bexarotene.

• Excessive bleeding
• Fainting or feeling lightheaded
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple punctures to locate veins

AACE Thyroid Task Force. American Association of Clinical Endocrinologists Medical Guidelines for Clinical Practice for the Evaluation and Treatment of Hyperthyroidism and Hypothyroidism. Endocr Pract. 2002;8(6) 459.