What are Thyroid Peroxidase (tpo) Antibodies?

Instructor: Darla Reed

Darla has taught undergraduate Enzyme Kinetics and has a doctorate in Basic Medical Science

In this lesson you will discover what thyroid peroxidase (TPO) antibodies are and what can be learned from them. You will also learn some about the role of thyroid peroxidase in the body.

What Is Thyroid Peroxidase (TPO)?

Have you ever gotten a cut or scrape? How did you treat it? Did you put some hydrogen peroxide on it to disinfect it?

Hydrogen peroxide (H2O2) is a popular disinfectant for cuts and scrapes.

Hydrogen peroxide structure
H2O2 structure

It comes in those brown bottles and stings like crazy.

Hydrogen peroxide bottle
Example of h2o2 bottle

You know it's working when it starts to bubble. But H2O2 isn't just for external use. Our bodies have other, internal uses for it.

One place H2O2 is used by the body is in the thyroid. The thyroid is a gland found in your neck that makes and secretes special hormones that have many different effects on your body. For example, the hormones secreted by the thyroid can affect growth and body temperature, among many other things.

Thyroid
Thyroid diagram

A peroxidase is an enzyme that can use hydrogen peroxide to function. Enzymes are proteins that speed up chemical reactions, so a peroxidase is an enzyme that can use H2O2 to speed up a chemical reaction. There is a special peroxidase found in the thyroid. That enzyme is called thyroid peroxidase.

Thyroid peroxidase (TPO) is an enzyme in thyroid cells that uses H2O2 to make thyroid hormones. It is found inside thyroid cells and on the cells' surfaces. TPO takes iodine (I) and changes it into an active molecule.

TPO converts I to active I-
TPO converts I to active I-

The active iodine attaches to a molecule called thyroglobulin. TPO then can take pieces of thyroglobulin that have the iodine attached and hook them together, like attaching two key rings together, and thus form the thyroid hormones.

TPO catalyzes formation of thyroid hormones.
TPO aids in the formation of thyroid hormones

What Are TPO Antibodies?

Have you ever accidentally scraped or scratched yourself? TPO antibodies can function kind of like these self-injuries. Your body usually uses the immune system to fight disease, but sometimes it can backfire and instead of fighting disease your body starts to fight itself in a condition called autoimmunity.

The immune system targets normal components in autoimmunity
Immune system fights the body in autoimmunity

One part of the immune system that helps fight disease is called an antibody. To understand how they work, think of play-dough. You can squish and shape it however you want. If you push your thumb into it, the play-dough will take the shape of your thumb. If you leave it out, play-dough will dry out and keep the shape of your thumb. Antibodies are like dried out play-dough that's been given a special shape.

Antibodies (Ab) are proteins with their own unique shape that attach to molecules called antigens (Ag) that fit that shape, just as your thumb would fit perfectly into the dried out play-dough, but your pinky wouldn't.

Ab and Ag
Antibodies and their antigens

Once attached to an Ag, the antibodies won't let go and they tell the immune system that the Ag is bad news, so the immune system goes after it and destroys it.

Sometimes the immune system gets hold of a molecule that is a normal part of your cell. When this happens, the immune system will start making antibodies to that normal molecule. In the case of TPO, the immune system is mistakenly given TPO as an Ag that needs to be destroyed.

Parts of TPO become Ag
Parts of TPO become Ag for Ab

TPO antibodies (TPO Ab), therefore, are Ab made by the immune system that grab and hold on to TPO and target it for destruction.

Ab bind TPO and target it for destruction
Antibodies bind part of TPO they recognize

Production and Effects of TPO Ab

There are many diseases caused by autoimmunity, and some of those diseases involve the thyroid. Hyperthyroidism and hypothyroidism are two diseases that can be caused by autoimmunity. In these diseases, your body produces high amounts of TPO Ab. One way doctors can check to see if you have hyper- or hypothyroidism is to find out if there are high amounts TPO Ab in your blood.

The presence of a lot of TPO Ab in your blood is a strong indicator that you could have a thyroid disease. It can also tell doctors that you are at risk for thyroid disease. For pregnant women, high amounts of TPO Ab in the blood can increase chances of a miscarriage. The presence of TPO Ab has been linked with post-partum depression and impaired intellectual development in children of mothers with TPO Ab as well.

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