Background: Thyroid stimulating hormone receptor antibody (TSHRa) assay has been in clinical use to diagnose Graves’ disease. Current TSHRa assay on the Roche automated platform does not differentiate between stimulating, neutral or blocking antibodies. Thyroid stimulating immunoglobulin (TSI) assay on the Siemens Immulite is supposedly specific for TSHR–stimulating autoantibodies.
Aim: To assess the sensitivity of the newly available Siemens Immulite TSI in the diagnosis and monitoring of Graves’ disease.
Method: This is a longitudinal study of 96 patients with newly diagnosed primary hyperthyroidism between February to June 2016. Residual sera/plasma collected for thyroid function tests (TFTs) were stored for TSI analysis. Only 30 patients who had follow-up TFTs were included in the analysis. Most of these samples had concurrent TSHRa requested at initial presentation. The cut-off for TSI assay was 0.55IU/L and that for TSHRa was 1.8 U/L.
Results: Of the 30 patients, 13 had three consecutive samples and 17 had two. Serum TSI fell with treatment in 26 patients (86.7%) and fell by ≥10% in 11 patients (36.7%). 10% (3) patients had TSI values above the detectable limit of the assay (>40IU/L). There was absolute concordance between TSHRa and TSI positivity. No correlation was found between the elevation of free T4/T3 and TSI titre neither did the rate of drop in free thyroid hormones predict the fall in TSI.
Conclusion: This study confirmed that TSI titre falls with anti-thyroid treatment as seen in our current TSHRa assay. However, to assess the rate of fall or treatment related resolution of TSI, further studies of more patients over longer time periods would be required. At this stage, TSI or TSHRa assays are comparable for routine use in the diagnosis of Graves’ disease.