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 makes what are 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 makes autoantibodies, which bind to healthy cells within the body, typically causing adverse effects. A thyroid stimulating immunoglobulin is one example of such an autoantibody, and it 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. An excessive release of thyroid hormone leads to a disease known as hyperthyroidism, or Grave's disease.
Normally, TSH, which is also known as thyrotropin, is released by the brain's pituitary gland in response to levels of thyroid hormone in the blood. When levels drop, the pituitary responds and more TSH is secreted, traveling in the blood to the thyroid gland, where it attaches to special TSH receptors on thyroid cells. This attachment stimulates the thyroid gland to grow and causes its cells to produce more thyroid hormone, which is released into the blood.
Once the pituitary detects that blood levels of thyroid hormone have risen, it stops making so much TSH. When a thyroid stimulating immunoglobulin attaches to the TSH receptor on a thyroid gland cell, like TSH it causes the gland to grow and the production of thyroid hormone to increase. The pituitary has no control over the amount of thyroid stimulating immunoglobulin in the blood, so TSI keeps stimulating the thyroid to make more and more thyroid hormone, leading to a condition known as Grave's disease.
In Grave's disease, abnormally high levels of thyroid hormone cause symptoms including tiredness, sweating, palpitations and anxiety. As 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 protrusion of the eyeballs and skin swelling, especially on the legs. Grave's disease may be treated using drugs that block thyroid hormone production, by destroying the thyroid gland with radioactive iodine, or by removing it surgically. When the gland is effectively removed, thyroid hormone production stops and a person then needs to take a hormone replacement for life.