This test measures the level of thyroid stimulating immunoglobulin, or TSI, in the blood. A doctor can use this test to find out what is causing hyperthyroidism, also known as Graves' disease. Hyperthyroidism occurs when too much thyroid hormone is secreted by the thyroid gland. When this happens, the body's metabolic rate increases to abnormally high levels.
TSI is an antibody that binds to special receptor sites on the thyroid gland that normally bind to the thyroid stimulating hormone, TSH. TSH is the hormone that stimulates the thyroid gland to secrete thyroid hormone. TSI mimics the effect of TSH, thereby causing the thyroid to secrete excess thyroid hormone.
This test is normally performed to diagnose and evaluate suspected thyroid disease. It may also be used to monitor treatment of certain thyroid disorders.
To measure the amount of thyroid stimulating immunoglobulin, TSI in the blood, a blood sample is taken from a vein on the forearm or hand. First, the skin over the vein is cleaned with an antiseptic. Next, a strong rubber tube, or "tourniquet," is wrapped around the upper arm. This enlarges the veins in the lower arm by restricting blood flow through them. A fine needle is gently inserted into a vein, and the tourniquet is removed. Blood flows from the vein through the needle, and is collected in a syringe or vial for testing in the laboratory. After the needle is withdrawn, the puncture site is covered for a short time to prevent bleeding.
A person should request specific instructions from his or her healthcare provider.
Normally, there is no TSI in the blood. If TSI is found in the blood, this indicates that the thyroid stimulating immunoglobulin is the cause of the of a person's hyperthyroidism.
Author:David T. Moran, MD
Editor:Keefe, Sandy, RN, MSN
Reviewer:Eileen McLaughlin, RN, BSN