Antithyroid microsomal antibody

Alternative names
Thyroid antimicrosomal antibody; Anti-microsomal antibody; Microsomal antibody

Definition
This is a test to measure antithyroid microsomal antibodies in the blood. Microsomes are small cell particles.

How the test is performed

Blood is drawn from a vein, usually on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to fill with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

For an infant or young child:
The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

How to prepare for the test
Fasting may be required for 6 to 8 hours before the test (usually overnight). Medications that affect the test will be monitored or discontinued during the test.

For infants and children:
The preparation you can provide for this test depends on your child’s age, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics:

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
This test is performed to confirm the cause of thyroid problems or other autoimmune disorders. The body produces microsomal antibodies in response to microsomes escaping from damaged thyroid cells. Such autoantibodies are usually present in Hashimoto’s thyroiditis. However, they can also be increased in other autoimmune disorders.

Normal Values

A negative test is normal.

What abnormal results mean

A positive test may indicate:

Additional conditions under which the test may be performed:

     
  • autoimmune hepatitis

What the risks are

     
  • excessive bleeding  
  • fainting or feeling light-headed  
  • hematoma (blood accumulating under the skin)  
  • infection (a slight risk any time the skin is broken)  
  • multiple punctures to locate veins

Special considerations

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.

Johns Hopkins patient information

Last revised: December 6, 2012
by Simon D. Mitin, M.D.

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