TSI (Thyroid-Stimulating Immunoglobulin): What Your Result Means

What is TSI (Thyroid-Stimulating Immunoglobulin)?

Thyroid-stimulating immunoglobulin (TSI) is an autoantibody that binds the TSH receptor and switches it on, forcing the thyroid to overproduce hormone. It is the defining driver of Graves' disease, the most common cause of hyperthyroidism. Testing TSI helps confirm Graves' when the diagnosis is unclear and helps monitor for relapse after treatment.

Typical reference interval: Reported as negative/positive or as a percentage of activity, with lab-specific cutoffs. A clearly positive result supports Graves' disease; a negative result makes it less likely but does not fully exclude it. Some labs use the related TRAb (TSH-receptor antibody) test instead.. Laboratories differ — the interval printed on YOUR report always governs.

If your TSI (Thyroid-Stimulating Immunoglobulin) is high

Common reasons TSI (Thyroid-Stimulating Immunoglobulin) runs above the reference interval:

• Graves' disease — the classic cause — TSI stimulates the thyroid continuously, producing hyperthyroidism
• Graves' eye disease — TSI contributes to the eye changes seen in some Graves' patients
• Relapse risk after treatment — persistently high TSI after antithyroid therapy predicts a higher chance of recurrence
• Neonatal Graves' disease risk — TSI crosses the placenta and can transiently affect a newborn's thyroid

If your TSI (Thyroid-Stimulating Immunoglobulin) is low

Common reasons TSI (Thyroid-Stimulating Immunoglobulin) runs below the reference interval:

• No Graves' activity — a negative result points away from Graves' disease — the expected finding in most people
• Other causes of hyperthyroidism — a negative TSI with hyperthyroidism suggests another driver, such as a toxic nodule

Symptoms worth knowing

TSI causes no symptoms itself. The hyperthyroidism it produces does: palpitations, weight loss, heat intolerance, anxiety, tremor, and sometimes bulging eyes. TSI is the laboratory explanation for those symptoms in Graves' disease.

What usually happens next

1. A positive TSI with hyperthyroid labs confirms Graves' disease — a clinician will plan treatment accordingly
2. Because TSI predicts relapse, it is sometimes tracked after treatment to guide long-term decisions
3. In pregnancy, a history of Graves' disease or positive TSI warrants fetal thyroid monitoring

Frequently asked questions about TSI (Thyroid-Stimulating Immunoglobulin) results