Antithyroid Microsomal Antibody

Written by Ann Pietrangelo
Medically Reviewed by George Krucik, MD

Antithyroid Microsomal Antibody

Antithyroid microsomal antibody is a test that measures antithyroid microsomal antibodies in your blood. Your thyroid is a gland in your neck that produces hormones that help regulate your metabolism. Your body produces these antibodies when your thyroid cells become damaged.

Your doctor may order this test in conjunction with other tests to help diagnose thyroid problems or other autoimmune conditions.

This blood test generally requires that you refrain from eating or drinking for six to eight hours before the test. A blood draw is a simple procedure that carries few risks. Actual testing of the blood takes place in a laboratory. Your doctor will discuss the results with you.

This test is also known as a thyroid peroxidase test.

How the Blood is Drawn


Be sure to inform your doctor about all the prescription and over-the-counter medications and supplements you take. Your doctor may instruct you to stop eating and drinking for six to eight hours prior to the antithyroid microsomal antibody test.


Your healthcare provider will choose a site on your arm, typically the back of your hand or the inside of your elbow, and clean it with antiseptic. An elastic band will be tightened around your upper arm to make your veins swell. This will make it easier to access the vein.

A needle will be inserted into your vein. You may feel a sting or prick as the needle is inserted. Some people report mild throbbing or discomfort. A small amount of blood will then be collected into a tube. Once the tube is filled, the needle will be removed. A bandage is usually placed over the puncture site.

For babies or young children, a sharp tool (lancet) is sometimes used for the skin puncture and the blood is collected onto a slide.

The blood sample will be sent to a laboratory for analysis and your doctor will discuss the results with you.

Risks and Side Effects

There are few risks or side effects associated with a blood test. Veins vary in size and, occasionally, your healthcare provider may have difficulty obtaining the sample.

Any time the skin is broken, there is a slight risk of infection. Other minimal risks include bleeding, bruising, light-headedness, and fainting.

What the Results Mean

You will probably get the results of your blood test in a few days. Your doctor will explain your specific results to you. A test that comes back as negative for antithyroid microsomal antibodies is considered a normal result.

These antibodies are usually not found in a healthy immune system. If you have an autoimmune disease or thyroid disorder, your antibody levels may rise. A positive test indicates an abnormal result and may be due to a variety of conditions, including:

  • Hashimoto’s thyroiditis (swelling of the thyroid gland that often results in reduced thyroid function)
  • granulomatous thyroiditis (also known as subacute thyroiditis, a swelling of the thyroid gland that usually follows an upper respiratory infection)
  • autoimmune hemolytic anemia (a drop in the number of red blood cells due to increased destruction by the immune system)
  • nontoxic nodular goiter (enlargement of the thyroid gland)
  • Sjogren syndrome (an autoimmune disorder in which the glands that produce tears and saliva are damaged)
  • Graves disease (an autoimmune disorder in which the thyroid gland is overactive)
  • systemic lupus erythematosus (a long-term autoimmune disorder affecting the skin, joints, kidneys, brain, and other organs)
  • rheumatoid arthritis
  • thyroid cancer

Women with high levels of antithyroid microsomal antibodies have a higher risk of miscarriage, preeclampsia, premature birth, and difficulty with in vitro fertilization.

It is important to note that having antithyroid antibodies in your blood does not always mean you have a thyroid disease. However, you may be at increased risk for future thyroid disease, and your doctor may want to monitor your condition.

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