TSI Test in California

What this test measures

Thyroid stimulating immunoglobulin is an autoantibody that targets the TSH receptor on thyroid cells. Instead of merely blocking the receptor, it activates it, prompting the gland to release more thyroid hormone. This activity distinguishes it from other thyroid antibodies and links it directly to the overactive thyroid seen in Graves disease. Measuring TSI gives a functional view of what the immune system is doing.

The test requires a blood sample drawn from a vein, usually at the arm, and no special preparation is typically needed. Laboratories use assays such as the thyrotropin receptor antibody or TSI bioassay to detect and often quantify the antibody. The reported value reflects the level of stimulating activity and is compared against a reference range established by the performing laboratory.

TSI testing is commonly ordered when a person has signs of hyperthyroidism, such as a rapid heartbeat, weight loss, heat intolerance, or an enlarged thyroid. It can help distinguish Graves disease from other causes of an overactive thyroid, including toxic nodules or thyroiditis. Providers may also use it to monitor disease activity or assess thyroid antibody status during pregnancy planning.

Results are not a stand-alone diagnosis. Antibody levels can fluctuate, and some people with Graves disease have low or undetectable TSI, while others without active disease may show low levels. A qualified healthcare provider interprets the result together with thyroid function tests, symptoms, and imaging to reach an accurate assessment.