MON-593 Thyrotropin Receptor Stimulating Immunoglobulin: Clinical Utility in Graves' Disease Diagnosis

Introduction: Thyroid stimulating hormone receptor autoantibodies (TRAb) are a diagnostic hallmark of Graves' disease (GD) and their clinical performance have been extensively reported in the medical literature. One of the three subtypes of TRAb is stimulating (S-TRAb), which leads to unregulated thyroid hormone production by activating the TSH receptor on thyroid follicular cells. The aim of our study was to assess the clinical utility and accuracy of the immunoassay Siemens/Immulite TSI, that supposedly measures only S-TRAb and compare it with Roche/Cobas method which measures total TRAb, in the diagnosis of GD. Methods: We performed a prospective study, evaluating serum samples of 107 patients, between January and September of 2018: 36 with untreated GD (group 1) and 76 control subjects [group 2 - 40 with euthyroid multinodular goiter (EMNG), 24 with autoimmune thyroiditis (AIT) and 12 with toxic multinodular goiter (TMNG)]. GD was diagnosed according to the American Thyroid Association guidelines. Statistical analysis was performed using independent samples t-test. Results: Median value of TRAb and S-TRAb in group 1 patients was 13.3 (cut-off <1.7 UI/L) and 7.3 (cut-off <0.55 UI/L), respectively, versus 0.4 and 0.12 in group 2, respectively. In group 2, patients with TMNG had higher TRAb levels than patients with MNG and AIT (0.96 versus 0.48 and 0.49, respectively; p=0.011), but similar levels of TSI (0.14, 0.16 and 0.1, respectively; p=0.246).By receiver operating characteristics curve analysis, we obtained a TRAb value of 1.75 UI/L (sensitivity 97.8%; specificity 97.4%) and S-TRAb value of 0.75 UI/L (sensitivity 100%; specificity 98.7%) as the best cut-off values to diagnose GD. S-TRAb levels were positive in all GD patients and negative in all but one control (sensitivity 100%; specificity 98.7%), whereas TRAb were positive in all but one patient with GD and negative in all but two controls (sensitivity 96.8%; specificity 98.1%). Discussion:There are very few studies regarding the diagnostic utility and accuracy of S-TRAb in GD diagnosis. It has a very similar diagnostic accuracy compared to TRAb, but it could be useful in particular cases, as it has a slightly higher sensitivity and specificity and can accurately detect very low values of analyte.