Thyroid Stimulating Immunoglobulin (TSI) Test

This blood test is used to evaluate levels of Thyroid stimulating immunoglobulin, also known as TSI, to determine if there is a deficiency and to help evaluate the thyroid. Thyroid stimulating immunoglobulin (TSI) is a form of immunoglobulin G (IgG) that is capable of binding to thyroid stimulating hormone (TSH) receptors. TSIs mimic the action of TSH, which causes excess secretion of thyroxine (T4) and triiodothyronine (T3).

What does a high TSI level mean?

The TSI level is abnormally high in those with hyperthyroidism due to Graves' disease. Graves' disease is a treatable autoimmune disease with symptoms that include: enlarged thyroid, weight loss and heat sensitivity. Graves' disease is much more common in women than men. Unusually high TSI levels can also be an indicator of Hashimoto's thyroiditis and, subsequently, hashitoxicosis (Htx), however Graves' disease is more commonly the culprit.