Thyroglobulin & Anti-Thyroglobulin Antibodies

Instructor: Darla Reed

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

This lesson will allow you to discover what thyroglobulin is, what it does, as well as describe its important role in the body. In addition you will learn what anti-thyroglobulin antibodies are and their clinical usage.

What is Thyroglobulin?

Have you ever flown on a plane? If so you probably went to the airport, got in line with other passengers, and then waited for the staff to let you on the plane. You have a gland called the thyroid that produces thyroid hormones in a way similar to a plane getting ready for takeoff.

Your thyroid is a gland in your neck made up of thyroid cells that surround your throat.

Thyorid
Thyroid diagram

Thyroid hormones help you grow, regulate your body temperature, heart rate and many other bodily functions. The thyroid cells have lots of protein in them, which are made up of amino acids, the building blocks of proteins. The particular amino acid needed to make thyroid hormones is called tyrosine (Tyr).

Tyrosine is an amino acid
Tyrosine is an amino acid

In order to make thyroid hormones, first iodine (I) has to be added to tyrosine. Tyrosine is like a plane waiting for passengers. Addition of I is like passengers boarding the plane.

Iodine is added to tyrosine
Iodide is added to Tyr

Once iodized, two tyrosine aminos combine to form a thyroid hormone.

Iodized tyrosine are combined to form thyroid hormones
Tyr with iodide added is combined with another Tyr plus iodide to form a thyroid hormone

Is tyrosine just floating around in the cell? No. They are part of a protein molecule called thyroglobulin. Thyroglobulin (Tg) is a glycoprotein, a big protein molecule with sugar added, that the cell uses to make hormones. Thyroglobulin is like the airport; it holds the tyrosine until it is iodized and combined.

Tg can contain iodized Tyr and thyroid hormones
Tg can contain iodized tyr and thyroid hormones

When the iodized tyrosine are combined to form a thyroid hormone, it is time for Thyroglobulin to let it go. The thyroid cell has special molecules that cut the ties holding the thyroid hormones to the thyroglobulin like a flight attendant closing and locking the cabin door while the plane separates from the terminal.

Special molecules release thyorid hormones and degrade Tg
Special molecules release thyroid hormones and degrade TG

Once the thyroid hormones are gone, the rest of the molecule is degraded, and the thryoid cell must make new iodized Tyrosine and thyroglobulin. The thyroid has lots of thyroglobulin and stores it and Iodine until it is ready to be used in making thyroid hormones.

Cells also release thyroglobulin into the lymph and it becomes a normal part of your blood plasma. Therefore, you will always have a low level of Tg in your blood and doctors can use thyroglobulin as a marker of thyroid activity to see how well your thyroid is functioning.

What are Anti-thyroglobulin Antibodies?

One way the immune system normally fights diseases is by making antibodies. Antibodies (Ab) are proteins that recognize specific parts of other molecules, kind of like keys are specific for their locks. Once an antibody finds its target molecule, it attaches and alerts the immune system that the target needs to be destroyed.

Ab bind specific targets
Ab bind specific targets

If a doctor tests you for thyroglobulin and the levels are above or below normal, it can indicate thyroid disease. One cause of thyroid disease is autoimmunity. Autoimmunity is when your immune system begins to fight your body instead of focusing on diseases.

In autoimmune thyroid disease, normal parts of the thyroid cell are singled out by antibodies and attacked like an innocent person convicted of a crime they didn't commit. Thyroglobulin is one of the normal thyroid cellular components that can be singled out by antibodies.

Tg can be a target during autoimmunity
Tg can be a target during autoimmunity

Anti-thyroglobulin antibodies (TgAb)_ or thyroglobulin antibodies are antibodies that have latched onto thyroglobulin as their target and marked it for destruction.

Ab target Tg
Ab target Tg

TgAb Production and Effect

In the normal course of making thyroid hormones, pieces of thyroglobulin are sometimes shown to the immune system, which then develop antibodies to them. Therefore, some low levels of anti-thyroglobulin antibodies can be in healthy people without always being found. In fact, TgAb have been seen to increase with age and in some cases has been correlated with decreased fragility in the elderly.

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