Thyroid Stimulating Immunoglobulin (TSI) Test

This test measures thyroid stimulating immunoglobulins in serum with an in vitro bioassay that detects the antibodies' ability to stimulate the TSH receptor. The result is reported as a percentage of baseline activity; Quest's reference value is under 140% of baseline. Unlike a binding assay, it specifically detects stimulating antibodies.

People with a low TSH and high thyroid hormones whose clinician wants to confirm whether Graves' disease is the cause; people with symptoms of an overactive thyroid such as a racing heart, tremor, heat intolerance, weight loss, anxiety, trouble sleeping or bulging eyes; people with Graves' disease being followed over time; and pregnant people with current or past Graves' disease, under their clinician's direction.

After thyroid function tests (TSH, free T4, free T3) suggest hyperthyroidism and the cause is not clear. In Graves' disease, clinicians may repeat antibody testing before stopping antithyroid medicine, since normal levels suggest a better chance of remission. In pregnancy, the ATA recommends antibody timing (for example at 18-22 weeks and late pregnancy) based on history; follow your clinician's schedule. No fasting is needed.

A result under Quest's reference value (under 140% of baseline) means stimulating antibodies were not detected at a significant level, which makes Graves' disease less likely, although it can be negative in very mild disease. A result above the reference value indicates thyroid-stimulating antibodies and, together with hyperthyroid blood tests, most often points to Graves' disease; less common causes include hashitoxicosis and, in newborns, neonatal thyrotoxicosis from maternal antibodies. Quest notes that markedly elevated TSI may be associated with a poorer response to antithyroid drug therapy. Review your result with a clinician or endocrinologist, who will interpret it with your TSH, free T4, free T3 and symptoms. JustLabs results are informational and are not a diagnosis.