Thyroid-stimulating immunoglobulin (TSI) is an autoantibody that can act on the thyroid gland, causing hyperthyroidism or Graves’ disease. TSI binds to the TSH receptor on thyroid cells, stimulating the growth of the gland and increasing the production of thyroid hormone. This can cause physical symptoms such as bulging eyeballs and swelling of the skin. Treatment options include drugs, radioactive iodine, or surgical removal of the thyroid gland.
A thyroid-stimulating immunoglobulin, or TSI, is a type of molecule created by the immune system, known as an autoantibody. When disease threatens the body, the immune system produces so-called antibodies, which bind to specific targets on abnormal cells or infectious agents and trigger reactions to destroy them. In what is called an autoimmune disease, the immune system produces autoantibodies, which bind to healthy cells within the body, usually causing adverse effects. A thyroid-stimulating immunoglobulin is an example of such an autoantibody and is able to act on the thyroid gland in the same way as a hormone called thyroid stimulating hormone, or TSH, which stimulates the thyroid to produce and release more thyroid hormone. Excessive thyroid hormone release leads to a condition known as hyperthyroidism or Graves’ disease.
Normally, TSH, also known as thyrotropin, is released from the pituitary gland of the brain in response to thyroid hormone levels in the blood. When levels drop, the pituitary responds and more TSH is secreted, which travels through the blood to the thyroid gland, where it binds to special TSH receptors on thyroid cells. This attachment stimulates the growth of the thyroid gland and causes its cells to produce more thyroid hormone, which is released into the blood.
Once the pituitary senses that blood levels of thyroid hormone have increased, it stops producing so much TSH. When a thyroid-stimulating immunoglobulin binds to the TSH receptor on a cell of the thyroid gland, such as TSH, it causes the gland to grow and increase the production of thyroid hormone. The pituitary has no control over the amount of thyroid-stimulating immunoglobulin in the blood, so IST continues to stimulate the thyroid to produce more and more thyroid hormone, leading to a condition known as Grave’s disease.
In Graves’ disease, abnormally high levels of thyroid hormone cause symptoms including fatigue, sweating, palpitations, and anxiety. Because a thyroid-stimulating immunoglobulin can bind to receptors in other tissues, such as those around the eyes or under the skin, this can lead to physical signs such as bulging eyeballs and swelling of the skin, especially on the legs. Graves’ disease can be treated by using drugs that block the production of thyroid hormones, by destroying the thyroid gland with radioactive iodine, or by surgically removing it. When the gland is actually removed, the production of thyroid hormones stops and a person must therefore be on a hormone replacement for life.
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