Health Encyclopedia

TSH

Definition

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).

Alternative Names

Thyrotropin; Thyroid stimulating hormone

How the test is performed

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.

How to prepare for the test

Your health care provider may ask you to stop taking drugs that may interfere with the test. Drugs that can affect TSH measurements include:

Amiodarone
Antithyroid medications
Dopamine
Lithium
Potassium iodide
Prednisone

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 signs of abnormal thyroid function (hyperthyrodism or hypothyrodism). It is also used to monitor treatment of these conditions.

Normal Values

Normal values are 0.4 - 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 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 3.0 mIU/L.

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

What abnormal results mean

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)

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 wit 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)

References

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.

Review Date:6/17/2008
Reviewed By:Elizabeth H. Holt, MD, PhD, Assistant Professor of Medicine, Section of Endocrinology and Metabolism, Yale University. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

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