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Antithyroid Microsomal Antibody


An antithyroid microsomal antibody test is also called a thyroid peroxidase test. It measures antithyroid microsomal antibodies in your blood. Your body produces these antibodies when cells in your thyroid become damaged. Your thyroid is a gland in your neck that makes hormones. These hormones help regulate your metabolism.

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

How Your Blood Is Drawn

A blood draw is a simple procedure that has few risks. Actual testing of your blood takes place in a laboratory. Your doctor will discuss the results with you.


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 before this test. This is called fasting. If you fast before a test, you may get more accurate results.


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 is then 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 stinging or pricking sensation 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 called a lancet is sometimes used for the skin puncture and the blood is collected onto a slide.

The blood sample is sent to a laboratory for analysis. Your doctor will discuss your results with you.

Risks and Side Effects

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

Any time your skin is broken, there’s a slight risk of infection. You should notify your doctor right away if the area of the blood draw swells or starts to produce pus.

Other minimal risks include:

  • bleeding
  • bruising
  • lightheadedness
  • dizziness
  • nausea

What the Results Mean

Blood test results are processed within a week. In some cases, doctors receive them within 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, which is a swelling of the thyroid gland that often results in reduced thyroid function
  • Graves’ disease, which is an autoimmune disorder in which the thyroid gland is overactive
  • granulomatous thyroiditis, or subacute thyroiditis, which is a swelling of the thyroid gland that usually follows an upper respiratory infection
  • autoimmune hemolytic anemia, which is a drop in the number of red blood cells due to increased destruction by the immune system
  • nontoxic nodular goiter, which is an enlargement of the thyroid gland with cysts called nodules
  • Sjogren’s syndrome, which is an autoimmune disorder in which the glands that produce tears and saliva are damaged
  • systemic lupus erythematosus, which is a long-term autoimmune disorder affecting your 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
  • difficulty with in vitro fertilization

False Results

Having antithyroid antibodies in your blood doesn’t automatically 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. For unknown reasons, the risk tends to be higher in women.

There’s also the possibility of false-positive and false-negative results. False positives from this test usually indicate a temporary increase in antithyroid antibodies. False-negative results mean that your blood test doesn’t reveal the presence of the antibodies when they’re actually there. You can also get a false negative if you didn’t fast or you’re on certain medications. Therefore, it’s important to follow all of your doctor’s orders when taking the blood test.

Next Steps

Your doctor will perform further diagnostic tests if antithyroid microsomal antibodies are found. These antibodies usually indicate an autoimmune disease. Other thyroid issues such as hypothyroidism will probably be ruled out from the start if you have these antibodies present. Your doctor may order an ultrasound, biopsy, and radioactive iodine test to narrow down your diagnosis. You’ll probably need blood testing every few months until your condition is under control. 


Content licensed from:

Written by: Ann Pietrangelo and Kristeen Cherney
Medically reviewed on: Nov 19, 2015: Nicole Galan, RN

This feature is for informational purposes only and should not be used to replace the care and information received from your health care provider. Please consult a health care professional with any health concerns you may have.
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