SLOT: Full Definition
What is TSI?
TSI, short for thyroid stimulating immunoglobulin, is a blood test that measures an antibody capable of binding to and activating the TSH receptor on the thyroid gland. Unlike normal TSH, which the pituitary regulates moment to moment, TSI is an immune protein that switches the receptor on and keeps it on — driving the thyroid into chronic overproduction of hormone.
TSI is the defining antibody of Graves' disease, the most common cause of hyperthyroidism. It is closely related to but slightly different from Trab — both are antibodies against the TSH receptor, but TSI specifically captures the stimulating subset, while TRAb is a broader umbrella that includes stimulating, blocking, and neutral receptor antibodies.
At Modern Thyroid Clinic, TSI is the test we order when Tsh is suppressed and free hormones are elevated, to confirm whether autoimmune Graves' disease is the driver.
Why does TSI matter?
Distinguishing Graves' disease from other causes of hyperthyroidism — toxic nodules, thyroiditis, factitious hyperthyroidism — changes how we treat. Graves' is autoimmune. Like Hashimoto's, it is driven by immune dysregulation that often responds to root-cause work alongside conventional therapy. Toxic nodules and thyroiditis are not autoimmune in the same way and require different management.
TSI helps answer:
- Is this hyperthyroidism truly Graves' disease?
- Is this autoimmune thyroid disease behaving in a stimulating rather than destructive direction?
- Is treatment lowering immune activity, or just managing hormone levels?
TSI can also be used to predict relapse risk and to guide pregnancy management in women with a Graves' history, since the antibody can cross the placenta.
Reference range
- Conventional lab reference range: typically less than 0.55 (varies by lab and assay; some labs use IU/L, others % activity, others SI units)
- Elevated TSI: confirms Graves' disease in the right clinical context
Unlike TSH or free T3, TSI is more of a yes-or-no question than a fine-tuned range. Either the antibody is present in clinically meaningful amounts, or it is not. The number can also help track immune activity over time and predict the likelihood of remission off antithyroid medication.
What does an abnormal TSI mean?
- TSI elevated with low TSH and high free T4 / free T3: Graves Disease. Confirms the diagnosis without the need for radioactive iodine uptake imaging in most cases.
- TSI elevated in remission or post-treatment: ongoing immune activity; higher risk of relapse.
- TSI elevated in pregnancy: important — the antibody can cross the placenta and affect the fetal thyroid, even if the mother is biochemically euthyroid on medication or after thyroidectomy.
- TSI normal with hyperthyroid labs: points away from Graves' and toward toxic nodules, thyroiditis, or other causes — confirm with Thyroid Ultrasound, Radioactive Iodine Uptake, and the full clinical picture.
At MTC, women with confirmed Graves' get conventional management coordinated with autoimmune root-cause care — gut, gluten, stress, sleep, nutrients — to support remission and reduce flare risk.
Common symptoms
Common questions
What's the difference between TSI and TRAb?
Both measure antibodies against the TSH receptor on the thyroid, but they capture slightly different things. **TRAb** is a broader umbrella test that detects all antibodies binding the TSH receptor — stimulating, blocking, and neutral. **TSI** is more specific: it measures only the **stimulating** subset, which is what drives Graves' disease. In practice, TRAb is often used as an initial screen because it is more sensitive, while TSI is preferred for confirmation, treatment monitoring, and pregnancy management. Many clinicians order both depending on availability and lab preference.
Can TSI go away with treatment?
Often yes. TSI commonly falls during antithyroid medication treatment, sometimes returning to non-detectable. Roughly 30 to 50% of patients achieve sustained remission of Graves' after a year or more of medical therapy, and TSI levels at the end of treatment help predict who is most likely to stay in remission. Persistent elevation after treatment, or a sharp rebound off medication, raises relapse risk. At Modern Thyroid Clinic we coordinate antithyroid management with root-cause work to support immune calming and improve the chance of durable remission.
Why do you check TSI in pregnancy?
TSI antibodies can **cross the placenta** and stimulate the fetal thyroid, even when the mother is biochemically euthyroid — including after thyroidectomy or radioactive iodine ablation, where she has no thyroid of her own to stimulate. High maternal TSI in pregnancy increases the risk of fetal or neonatal hyperthyroidism, which is why obstetric and endocrine guidelines recommend measuring it in any woman with current or past Graves' disease. The result guides both maternal monitoring and fetal surveillance during the pregnancy.
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