Rapid and sensitive bioassay for thyroid-stimulating immunoglobulin detection in TBII-negative euthyroid Graves’ orbitopathy

Objective Graves’ orbitopathy (GO) typically occurs in hyperthyroid patients with detectable TSH-receptor antibodies (TSH-R-Ab). Rarely, it presents in euthyroid patients without detectable TSH-R-Ab, as determined by thyrotropin-binding inhibitory immunoglobulin (TBII) assays, posing a diagnostic challenge. While functional bioassays improve TSH-R-Ab detection in euthyroid GO (EGO), their complexity limits clinical use. A novel, highly sensitive bioassay (Turbo TSI) offers a faster and simplified protocol. In this study we explore Turbo TSI in TBII-negative EGO and compare it to two distinct types of binding immunoassays. Furthermore, we describe a large cohort and compare clinical characteristics to hyperthyroid GO patients. Methods Clinical features, thyroid function and TBII levels of 20 cases were recorded retrospectively. If available, biobank serum was used to reassess TSH-R-Ab levels with Turbo TSI bioassay and bridge-based TSI binding immunoassay. TBII-negative EGO patients were compared to 40 hyperthyroid GO controls. Results All cases were TBII-negative and euthyroid at presentation. From six patients, biobank serum was available. Two of these sera were found positive with bridge-based TSI binding immunoassay. In contrast, all six sera were found positive with the Turbo TSI bioassay. Compared to hyperthyroid GO, TBII-negative EGO patients presented unilaterally/asymmetrically more often, had a milder and less active phenotype and required less surgical intervention. Conclusions TSH-R-Ab are not absent in TBII-negative EGO, but require highly sensitive bioassays for detection. In contrast to traditional bioassays, Turbo TSI offers a simple and fast protocol and can therefore facilitate diagnosis in these patients. EGO presents with a milder phenotype than hyperthyroid GO.