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Thyroid-Stimulating Immunoglobulin (TSI) Activity, Serum

Published: August 9, 2026Updated: August 16, 2026

The TSI test measures thyroid-stimulating activity in the blood. It is mainly used to help diagnose and monitor Graves disease.

Quick summary

["An elevated TSI result can support a diagnosis of Graves disease, an autoimmune cause of hyperthyroidism."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
30166-3
Short code
TSI Act/Nor Ser
Synonyms
TSI Act/Nor Ser
Category
thyroid
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Specimen volume
5 mL
Stability
Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
Hemolysis max
500
Lipemia max
1000
Icterus max
500
TAT
4–24 hours
Order
routine

What is this test?

The thyroid-stimulating immunoglobulin (TSI) test detects antibodies that can stimulate the thyroid gland's TSH receptor. It may be performed as an activity or functional assay, depending on the laboratory.

Why might I need this test?

This test may be ordered when symptoms or other laboratory results suggest an overactive thyroid. It can help identify Graves disease and may also be used to monitor the effect of treatment.

How is the test performed?

A blood sample is collected from a vein and tested in the laboratory. The blood is usually collected in a plain or serum-containing tube, with no special handling required by the patient.

How do I prepare for the test?

No specific preparation is usually necessary. Tell your clinician if you are taking thyroid medicines or other treatments that could affect thyroid-test results.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

An elevated or positive TSI result can indicate the presence of thyroid-stimulating antibodies and supports a diagnosis of Graves disease when interpreted with symptoms and other thyroid tests. A result that is not elevated makes Graves disease less likely but does not always exclude it, particularly after treatment or in rare antibody-negative cases.

Short name

TSI test

Frequently asked questions

What does a positive TSI test mean?
A positive or elevated result means thyroid-stimulating antibodies are present. This is most often associated with Graves disease, but the result must be interpreted with your other thyroid tests and clinical findings.
Is the TSI test the same as a TSH test?
No. TSH measures the amount of thyroid-stimulating hormone in the blood, while TSI measures antibodies that can imitate TSH and activate the thyroid.
Does a negative TSI result rule out Graves disease?
Usually, a negative result makes Graves disease less likely, but it does not completely rule it out. Your clinician may repeat the test or use other tests if the clinical picture still suggests Graves disease.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 16, 2026. · Read our editorial policy