What Is a Thyroid Peroxidase Antibody Test?

Thyroid peroxidase (TPO) is an enzyme made by your thyroid, the butterfly-shaped gland located in the front of your neck. The thyroid gland uses TPO along with iodine to generate the thyroid hormones triiodothyronine (T3) and thyroxine (T4). Together, these hormones regulate body temperature, heart rate, and metabolism. They also promote growth, development, and maturation of the brain and nervous system.

Antibodies serve a purpose in your body; they defend against bacteria, viruses, and toxins. In an individual with a healthy functioning immune system, the body views TPO as an essential, hormone-making enzyme. However, in individuals with an autoimmune condition, the body can form proteins called antibodies, which attack healthy tissues and organs, including the thyroid.

If you have an autoimmune thyroid disorder, your blood levels of TPO antibodies can increase and can be detected with a simple blood test. Further testing may be needed to determine the cause and offer a diagnosis.

Purpose

The TPO antibody test is done to help diagnose Hashimoto’s thyroiditis. The symptoms that may be indicative of Hashimoto’s thyroiditis include:

• Fatigue
• Menstrual irregularities
• Dry skin
• Brittle nails
• Constipation
• Cold intolerance
• Muscles aches and pains
• Depression
• Brain fog

Additionally, your healthcare provider might order other thyroid tests in conjunction with the TPO antibody test, which could include thyroglobulin antibodies, thyroid-stimulating hormone (TSH), free T4 hormone, and T3 hormone.  

Risks and Contraindications

The TPO antibody test is considered safe, and the risks associated with it are mild. If you’ve had a previous blood sample taken from your arm, this test will be similar. Veins differ from person to person. In some cases, your nurse or technician may have trouble finding a vein and may need to insert the needle more than once to collect blood. This may cause slight pain.

Also, you might experience bruising or a stinging sensation at the insertion of the needle—bruising may be alleviated or minimized by keeping a bandage in place for the amount of time the clinician recommends following the blood draw. The discomfort should disappear in a day or two. On a rare occasion, a vein may swell and cause a condition known as phlebitis, which is often treated using warm compresses throughout the day.

Sometimes, people may feel faint or lightheaded with blood tests. If this describes you, be sure to let the technician know so that you can remain seated in the chair for a couple of minutes before getting up. You may also have the option of taking the test in a lying position. 

Before the Test

Since the TPO antibody test may be done with other thyroid tests, your healthcare provider may ask you to stop taking medications that could affect your thyroid levels. These may include thyroid replacement drugs, steroids, certain heart medications, and others.

Your healthcare provider will provide you with specific instructions for taking this blood test. You could be asked to avoid eating and drinking for several hours before having your blood drawn; often, this is most conveniently done overnight before the test.

On the day of the test, you’ll be able to wear your normal clothes. But to make it easier for the technician to access your veins, you may want to consider wearing a shirt with sleeves you can roll up. Also, be sure to bring your insurance card and a form of identification with you so that the healthcare facility can bill your insurance carrier.

Remember that reimbursement for testing varies among insurance companies, so you may need to speak to your carrier to determine if you need pre-approval when doing a TPO antibody test.

During the Test

Typically, a lab technician will ask you to sit in a chair while one arm rests in front of you. Then, the lab technician will place an elastic band around this arm to restrict blood flow for a short time to locate a vein. After that, they will disinfect the area with an alcohol swab or pad.

Once the area has been disinfected, they’ll insert the needle into your arm and they’ll attach a tube that collects the blood at the base of the syringe. When enough blood has been drawn for the test, the lab technician will remove the elastic band, take out the needle, and place a bandage over the location of the puncture. Usually, this type of blood test only takes a few minutes.

After the Test

Following the test, your healthcare provider will provide you with any particular follow-up instructions when necessary. For example, if you had to fast before the test or skip medications, your healthcare provider will let you know if it’s okay to resume eating and taking your prescriptions.

Unless otherwise advised, you should be to go back to your normal activities. Generally, you’ll have the results of the tests in a couple of days.

Interpreting the Results

When there are no antibodies found in your blood, this is considered a negative and normal test result. If your test is positive, meaning antibodies were present in your blood, this could indicate that there is an autoimmune condition affecting your thyroid gland. If the test is negative, it is less likely that you have Hashimoto's disease.

It is only when the TPO antibodies are compared to other thyroid tests, most especially TSH, that a healthcare provider can make a more definitive diagnosis. Even so, if the disease is subclinical (with no overt symptoms) or the TSH is borderline normal, some experts discourage TPO antibody testing. Doing so may lead you to believe that you have autoimmune thyroid disease when you don't and divert you from the real cause of your symptoms.

Most likely, your healthcare provider will view your tests as one part of a larger picture, which includes other thyroid lab results, your symptoms, and your medical history.

A Word From Verywell

If you receive a positive test result from a TPO antibody test, your first reaction might be to feel a bit nervous. But remember, there are several reasons you may have a positive test, and you have many treatment options available to you. To put yourself at ease, make sure you have a healthcare provider whom you trust. A trusted provider can answer any questions and concerns so that together, both of you can make the best decision for your health.