Antithyroid microsomal antibody is a test to measure antithyroid microsomal antibodies in the blood. Microsomes are found inside thyroid cells. The body produces antibodies to microsomes when there has been damage to thyroid cells.

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.

You may be told not to eat or drink for 6 - 8 hours before the test (usually overnight). Your health care provider may monitor or advise you to stop taking medications that affect the results until after the test.

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.

This test is done to confirm the cause of thyroid problems, including Hashimoto's thyroiditis.

The test may also be used to diagnose other autoimmune disorders.

A negative test is normal.

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

A positive test may be due to:

• Autoimmune hemolytic anemia
• Granulomatous thyroiditis
• Hashimoto's thyroiditis
• Nontoxic nodular goiter
• Rheumatoid arthritis
• Sjogren syndrome
• Systemic lupus erythematosus
• Thyroid cancer

Other conditions under which the test may be performed:

• Autoimmune hepatitis

High levels of these antibodies have also been linked to an increased risk of:

• Miscarriage
• Preeclampsia
• Premature birth
• In vitro fertilization failure

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)

Mecacci F, Parretti E, Cioni R, et al. Thyroid autoimmunity and its association with non-organ-specific antibodies and subclinical alterations of thyroid function in women with a history of pregnancy loss or preeclampsia. J Reprod Immunol. 2000 Feb;46(1):39-50.

Ladenson P, Kim M. Thyroid. In: Goldman L and Ausiello D, eds. Goldman: Cecil Medicine. 23rd ed. Philadelphia, Pa:Saunders; 2007:chap 244.