Thyroid Stimulating Immunoglobulin
Thyroid stimulating immunoglobulin (TSI) is an antibody that mimics TSH and stimulates your thyroid to make hormone. This test detects the antibody most associated with Graves' disease.
Unlike antibodies that damage the thyroid, TSI activates it, driving excess hormone production. Detecting it helps confirm Graves' disease as a cause of an overactive thyroid. It can also be useful during pregnancy, since these antibodies can affect the baby.
A positive or elevated TSI supports Graves' disease as the cause of an overactive thyroid. Higher levels may correlate with more active disease, and monitoring can guide management. Your clinician interprets it alongside TSH and thyroid hormone levels.
A negative or normal result makes Graves' disease less likely as the explanation for thyroid overactivity, though other causes may still need evaluation.
TSI is reported relative to a lab-specific cutoff, with negative results considered the norm in people without Graves' disease. Your clinician can interpret the value in your specific context.
Sources
Educational context only, not medical advice or a diagnosis. Reference ranges vary by lab, age and sex; always discuss your results with a clinician.