LBSAT244 Prospective, Blinded, Multicenter Head-to-head Clinical Comparison Of Two "TSI" Assays In Patients With Autoimmune Thyroid Disease

OBJECTIVE: This study evaluated the clinical performance of the thyrotropin receptor Antibody (TSHR-Ab) bridge assay (Siemens TSI assay) versus the cell-based bioassay for the measurement of thyroid stimulating immunoglobulins (TSI, Quidel) (in patients with autoimmune thyroid disease (AITD). METHODS: Two-hundred two subjects (median age 54 years, 162 female, 80%), with well-documented thyroid disorders and controls were prospectively enrolled in a consecutive, unselected manner. Antibody measurements were performed using the Bridge (Siemens Immulite) and stimulating (TSAb or TSI) and blocking (TBAb) TSHR-Ab cell-based bioassays (Quidel Thyretain) each according to the manufacturers’ specfications. The testing laboratories were blinded to any clinical data. RESULTS: The three assays were negative in controls (n=10), patients with euthyroid nodular disease (n=11) and non-autoimmune thyrotoxicosis (n=21). In patients with Graves’ disease (GD), independent of disease duration or activity, TSHR-Ab positivity was present in 70/113 (62%) and 88 (78%) for the Bridge and TSAb bioassay, respectively (p<0. 001). Concordant positive results in the Bridge and TSAb bioassay were noted only in 67/88 (76%) patients with GD and TSAb positivity. Of these 21 discordant findings of TSAb positive and Bridge negative patients, four were new diagnosis of untreated overt Graves’ hyperthyroidism. Nine patients were euthyroid on low dose methimazole - two of these nine were taken off methimazole shortly after enrollment and remain in remission however the other seven required anti-thyroid drug (ATD) for at least 12 months post-enrollment. Three patients were euthyroid without treatment (follow-up data not available). Four had subclinical hyperthyroidism due to GD and one with GD and subsequent spontaneous hypothyroidism had moderate Graves’ orbitopathy (GO). Patients with GO were TSHR-Ab positive in 23/25 (92%) and 20 (80%) in the TSAb bioassay and Bridge assays, respectively (p<0. 001). In 160 patients with AITD, TBAb were present in 32 (20%), HT 11/27 (41%) and GD 21/133 (16%). 28/32 (88%) of the TBAb positive samples were also Bridge assay positive but TSAb bioassay negative. CONCLUSIONS: Thyroid Stimulating Immunoglobulin (TSI or TSAb) is the key clinical biomarker for GD. We found that the TSAb functional bioassay was more sensitive than the Bridge TSH-R-Ab test not only in cases of subclinical GD but also new overt GD hyperthyroidism, as well as in euthyroid patients on low dose ATD which impacts decisions regarding discontinuing therapy. TSAb was also a better biomarker in patients with GO. In patients with AITD and TSAb negative, the Bridge assay was positive in the presence of blocking TSHR-Ab measured in a TBAb bioassay. Presentation: Saturday, June 11, 2022 1:00 p.m. - 3:00 p.m.