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Thyroglobulin is a type of protein found inside the thyroid gland. Sometimes referred to as Tg for short, it is essential for the production of thyroid hormones, as parts of its structure are used to make them. In a condition known as Hashimoto's disease, people develop antibodies against thyroglobulin, destroying it and leading to a lack of thyroid hormone.
A normal thyroid gland is formed from many rounded shapes, called follicles, with a substance called colloid stored inside them. The wall of each follicle is made from a single layer of thyroid cells. Colloid is largely composed of thyroglobulin which has been made by these thyroid cells.
The thyroglobulin molecule acts as a kind of scaffolding for the manufacture of thyroid hormones. It is made up of numerous smaller units, called tyrosines. Iodine is taken up by the thyroid gland and moves into the colloid, where it attaches to some of the tyrosines at specific points to form what are called iodotyrosines. Pairs of iodotyrosines then join together to make thyroid hormones, although they still remain attached to the "scaffolding" at this point.
Eventually, the thyroid cells lining the follicle absorb parts of the colloid and digest it, breaking the thyroglobulin apart and releasing the thyroid hormones. The hormones move out of the cells into the blood, where they are taken to different parts of the body. There are two different thyroid hormones: thyroxine, or T4, and triiodothyronine, or T3. They act on cells throughout the body, affecting the metabolic rate, development and growth.
In a condition known as Hashimoto's disease, or Hashimoto's thyroiditis, the body itself attacks the thyroid. Typically, the immune system creates antibodies which target different parts of the gland, including thyroglobulin. Production of thyroid hormones is decreased, resulting in what is called hypothyroidism, where the body's functions slow down. This leads to symptoms such as fatigue, weight gain, feeling cold and having dry skin. Although the thyroid cannot be repaired, the condition can be treated by taking a synthetic or natural substitute to replace the missing hormones.
When thyroid cancers occur, the level of thyroglobulin in the blood is often raised, so levels may be monitored after treatment to check whether the cancer has been fully removed. Thyroglobulin levels are also monitored for a period of time following cancer treatment, as an increase could indicate that the cancer is returning. A number of other, non-cancerous, diseases may cause levels to rise, such as inflammation of the thyroid, and the presence of antibodies can also affect measurements.