Thyroid Antibodies: What a TPO Test Measures

The thyroid is a butterfly-shaped gland in the lower front of the neck. It makes thyroid hormones, which help the body use energy, stay warm and keep the brain, heart, muscles and other organs working (American Thyroid Association).

Thyroid antibody tests are blood tests that measure the level of thyroid antibodies in the blood (MedlinePlus). This guide explains, in general terms, what these antibodies are and what the tests measure.

What antibodies are

Antibodies are proteins the immune system makes to fight germs such as viruses and bacteria. Sometimes the immune system makes antibodies that attack the body's own healthy tissue by mistake. These are called autoantibodies (MedlinePlus).

Thyroid antibodies are a type of autoantibody. They target proteins that help the thyroid make its hormones (MedlinePlus).

Common thyroid antibodies

The American Thyroid Association describes thyroid peroxidase antibody and thyroglobulin antibody as two common thyroid antibodies (American Thyroid Association). MedlinePlus lists these and two others (MedlinePlus):

Thyroid autoimmunity in context

Hashimoto's disease is an autoimmune disorder that can cause hypothyroidism, or an underactive thyroid. It's also called Hashimoto's thyroiditis, chronic lymphocytic thyroiditis or autoimmune thyroiditis. It's the most common cause of hypothyroidism, which affects about 5 in 100 Americans (NIDDK).

According to NIDDK, Hashimoto's disease is 4 to 10 times more common in women than in men, and it most often develops between ages 30 and 50 (NIDDK). Graves' disease is the most common cause of hyperthyroidism, or an overactive thyroid (MedlinePlus).

Antibodies and thyroid function are measured separately

Thyroid antibody tests and thyroid function tests measure different things. The American Thyroid Association explains that TSH (thyroid stimulating hormone) and free T4 are what show how much thyroid hormone the body has. Antibody tests look for signs of thyroid autoimmunity instead (American Thyroid Association).

The ATA also notes that while antibody tests can help identify the cause of a thyroid problem, following antibody levels over time isn't helpful for tracking thyroid function (American Thyroid Association).

When to see a clinician

Questions about thyroid health, or about any change in health, are best discussed with a licensed clinician who knows a person's history. In a medical emergency, call 911 or go to the nearest emergency room.

Frequently asked questions

What does TPO stand for? TPO stands for thyroid peroxidase, a thyroid protein. Thyroid peroxidase antibodies are often shortened to TPOAb or anti-TPO (MedlinePlus).

Is Hashimoto's disease the same as hypothyroidism? No. Hashimoto's disease is an autoimmune disorder that can cause hypothyroidism. It's the most common cause of hypothyroidism, but not the only one (NIDDK).

What other names does Hashimoto's disease have? It's also called Hashimoto's thyroiditis, chronic lymphocytic thyroiditis or autoimmune thyroiditis (NIDDK).

How are thyroid antibody tests different from a TSH test? A TSH test reflects how the thyroid is functioning. Antibody tests look for signs of thyroid autoimmunity, which can help explain the cause of a thyroid problem (American Thyroid Association).