What does the Thyroid Stimulating Immunoglobulin (TSI) test measure?

Why is Thyroid Stimulating Immunoglobulin (TSI) measured?

TSI or TRAb is the pathognomonic marker of Graves' disease — the most common cause of hyperthyroidism — and is essential for distinguishing Graves' disease from other causes of hyperthyroidism (toxic multinodular goitre, toxic adenoma, thyroiditis) when the clinical picture is not diagnostic or imaging is unavailable. In a comprehensive thyroid panel, TSI provides the autoimmune hyperthyroidism aetiology marker that complements Anti-TPO and Anti-TG in the autoimmune thyroid disease picture. TSI positivity has critical implications in pregnancy — maternal TSI crosses the placenta and can cause foetal and neonatal Graves' thyrotoxicosis requiring neonatal monitoring. TSI normalisation is also used as a criterion for assessing remission potential before anti-thyroid drug withdrawal in Graves' disease.

Reference ranges

Standard reference

<140% baseline (negative); ≥140% baseline (positive) # REVIEWER FLAG: Confirm whether TSI (or alternatively TRAb — thyrotropin receptor antibody) is included in the Access Medical Laboratories Thyroid Panel Comprehensive, and which assay format is used. TSI (bioassay measuring cAMP stimulation) and TRAb (competitive binding immunoassay measuring total TSH receptor antibody) are related but distinct assays — confirm which is included, the specific reference range, and unit convention before publication. If neither is included in the standard panel, remove this biomarker entry and note that TSI/TRAb is available as a standalone add-on.

Functional medicine optimal

Not applicable — qualitative negative/positive marker; negative (<140% baseline) is the expected finding in all non-Graves' thyroid conditions.

Units

% baseline, IU/L

Reference ranges vary by laboratory and assay platform. Interpret in context of the issuing laboratory's stated range.