Thyroid Stimulating Immunoglobulin (TSI)
Antibodies that stimulate the thyroid by acting like TSH. A positive result is strongly linked to Graves' disease.
AI-verified against cited sources. Awaiting expert reviewAlso called TSI, TSAbLOINC 30567-2
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What it is
Your pituitary gland makes TSH, which tells your thyroid to make hormone. In Graves' disease, the immune system makes antibodies that attach to the same receptor and switch the thyroid on, so it makes too much hormone. These are called thyroid stimulating immunoglobulins, or TSI. This test measures how much of them is in your blood. Unlike the TRAb test, it looks only for the antibodies that stimulate the thyroid, not for ones that block it.
Why it matters
A positive TSI result is strongly linked to Graves' disease, the most common cause of an overactive thyroid. In the manufacturer's validation study, the test found Graves' disease in 98.6% of people who had it and correctly gave a negative result in 98.5% of people with other thyroid or autoimmune diseases. It is mainly used when the cause of an overactive thyroid is unclear, when someone with Graves' disease is planning pregnancy or is pregnant, and to follow treatment. It is not a test of how well your thyroid is working. TSH and free T4 do that.
If it’s high
A result above your lab's cutoff means stimulating antibodies were found. This is strongly linked to Graves' disease. How high the number is does not, on its own, tell you how overactive the thyroid is, and your clinician reads it with your TSH and thyroid hormone levels.
Common causes
- Graves' disease, the most common cause of an overactive thyroid
- Past Graves' disease: TSH receptor antibodies can stay detectable for some time after treatment
- Rarely, a positive result in people without Graves' disease: about 1.5% of people with other thyroid or autoimmune diseases tested positive in the manufacturer's study
What can help
- Ask how this result fits with your TSH, free T4 and free T3
- Ask whether a TRAb test is also needed, since the two often give the same answer
- If you are pregnant or planning to be, or have had Graves' disease, tell your prenatal clinician and ask how the antibodies will be followed
- If you are on treatment, ask when the antibodies will be rechecked
Usual ranges
What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.
For adults: 0–0.55 IU/L. Source: Labcorp, LabCorp Burlington (Extended Catalog), Journal of Clinical Endocrinology & Metabolism.
- Adults (this one applies)0–0.55 IU/L
Sources: Labcorp, LabCorp Burlington (Extended Catalog), Journal of Clinical Endocrinology & Metabolism
If this applies to you
TSI means a little more for some people. Open the one that fits you.
Thyroid condition
TSI tests are meant to pick up the stimulating kind of TSH receptor antibody, the kind that switches the thyroid on. The European guideline says cell-based tests for stimulating antibodies help predict thyroid problems in a baby during pregnancy, and effects of Graves' disease outside the thyroid, such as eye changes.
What can affect your result
- Test type: a bridge immunoassay reports IU/L, while a cell-based bioassay reports percent of a reference sample, and the two cannot be compared directly
- Treatment for Graves' disease, which can change antibody levels over time
- Pregnancy, when TSH receptor antibody levels tend to fall
- Heterophile antibodies or a very fatty (lipemic) sample
- Heterophile antibodies in the blood can react with the assay's components and interfere with the result (Labcorp).
- Labcorp rejects grossly lipemic samples.
- Results from the bridge immunoassay (IU/L) and the cell-based bioassay (percent) use different units and cutoffs and are not interchangeable.
- Treatment for Graves' disease can change antibody levels over time, so the timing of the draw relative to treatment matters.
Levels can change slowly over months of treatment, and are followed to help judge the chance of remission or relapse (Labcorp).
When to talk to a clinician
A positive result is not urgent by itself. Talk with a clinician about what it means alongside your TSH and thyroid hormones, especially if you have a fast or uneven heartbeat, shakiness, unplanned weight loss, heat intolerance, bulging eyes or swelling at the front of the neck. If you are pregnant and have a history of Graves' disease, tell your prenatal clinician about this result. Call 911 or your local emergency number for chest pain, trouble breathing, confusion or fainting.
Questions you could ask
- Which test method does my lab use, and what is its cutoff?
- Will the same lab and method be used each time, so my results can be compared?
- What does this result mean for my thyroid, given my TSH and thyroid hormone levels?
- Do I also need a TRAb test, or does this one give the same answer?
- How often should we recheck the antibodies while I am being treated?
- Does this matter for pregnancy plans?
Questions people ask
What is the difference between TSI and TRAb?
TSI looks for the antibodies that switch the thyroid on. TRAb is a binding test that finds antibodies attached to the TSH receptor, whether they switch the thyroid on or block it. In a large lab review the two tests agreed most of the time, so ordering both is often unnecessary.
Why does my lab report TSI in a different unit than another lab?
Some labs use a bridge immunoassay that reports IU/L. Others use a cell-based test that reports a percentage of a reference sample. The numbers are on different scales and cannot be converted, so compare your result with the cutoff on your own report.
Does a negative TSI rule out Graves' disease?
No. A negative result makes it less likely, but the test is not perfect. In the manufacturer's study it found Graves' disease in 98.6% of people who had it. Your clinician reads it together with your TSH, thyroid hormones and symptoms.
Show the science and sources
For clinicians and researchers
TSI are IgG autoantibodies that bind the TSH receptor and stimulate cAMP signaling without negative feedback. This is the cause of hyperthyroidism in Graves' disease. Two kinds of assay exist. The Siemens IMMULITE 2000 TSI is a semi-quantitative bridging immunoassay that uses TSHR chimeras, with the capture chimera modified to remove the epitope for inhibitory antibodies. It is traceable to WHO NIBSC 08/204, and its cutoff is 0.54 to 0.55 IU/L (Labcorp; Nelson 2025). In the validation, 361 people with Graves' disease and 404 people with other thyroid or autoimmune diseases showed 98.6% sensitivity and 98.5% specificity, with the highest non-Graves' result at 0.39 IU/L. Cell-based bioassays use CHO cells expressing the TSH receptor and report cAMP production as a percentage of a reference sample, with a negative cutoff of 122% or less in the laboratory described by Nelson 2025 (sensitivity 92.9%, specificity 99.2%). TRAb binding assays do not distinguish stimulating from blocking antibodies (Labcorp; ETA 2018), and ETA 2018 notes that functional bioassays help predict extrathyroidal manifestations and fetal or neonatal thyroid problems. In 189,000 patient encounters at a national reference lab, TRAb and TSI were ordered together in 14.3% to 17.4% of encounters, were discordant in 12.5% (bioassay) and 6.6% (bridge assay) of pairs, and the TSI assays aligned better with hyperthyroidism and confirmed Graves' disease (Nelson 2025). The authors concluded paired orders are mostly redundant. Labcorp lists the use of TSI for the cause of thyrotoxicosis, monitoring treatment, predicting remission or relapse, predicting neonatal hyperthyroidism, and confirming Graves' ophthalmopathy. TSH receptor antibodies can persist for a varied time after definitive treatment and tend to fall during pregnancy (Bucci 2017, for TRAb).
LOINC 30567-2 (primary), 17708-9. Specimen: serum.
Sources
- Thyroid Stimulating Immunoglobulin (TSI) (test 140749), Labcorp
- Thyroid-stimulating Immunoglobulin (TSI) (FFTSI), LabCorp Burlington (Extended Catalog)
- TSH Receptor Antibody Test Utilization Patterns From a National Reference Laboratory: TRAb, TSI, or Both?, Journal of Clinical Endocrinology & Metabolism (2025)
- 2018 European Thyroid Association Guideline for the Management of Graves' Hyperthyroidism, European Thyroid Journal (European Thyroid Association) (2018)
- Thyroid-Stimulating Hormone Receptor Antibodies in Pregnancy: Clinical Relevance, Frontiers in Endocrinology (2017)
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.