Endocrine Abstracts

Background: Autoantibody mimicry of hormone action at the thyrotropin receptor (TSH-R) and aberrant signaling of TSH-R by autoantibodies (TSH-R-Ab) causes autoimmune thyroid disease (AITD), hyperthyroidism and hypothyroidism, both of which affect millions of patients worldwide. In this multicenter, single blind study, the specificity, sensitivity and performance of a novel bioassay for stimulatory TSH-R-Ab were tested.

Methods: TSH-R-Ab were measured in a blinded manner with a TSH-R binding immunoassay (Cobas e411, Roche), and both stimulatory (TSAb) and blocking (TBAb) bioassays with luciferase release as readout (Thyretain®, Quidelortho) according to manufacturer’s instructions. TSH-R-Ab was also measured using the new TSAb bioassay (TurboTM). TurboTM utilizes a cAMP biosensor that is an engineered form of firefly luciferase in which the cAMP-binding domain of protein kinase A is fused between the N- and C-termini such that the luciferase is inactive until cAMP binds to the cAMP binding site. Using this biosensor, luciferase activity is proportional to intracellular cAMP levels. TurboTM detects TSAb in serum at room temperature, in a real-time homogeneous format, which delivers results in 60 minutes. It does not require cell culture, sample dilution, washing or cell lysis steps.

Results: Eight-hundred forty-four samples of unselected, consecutive patients with AITD and control subjects, devoid of autoimmune endocrine and thyroid disorders, were included. Mean age (SD) was 49 (14.7) years, range 1-89 years. Female: male sex ratio was 3.7:1. The four TSH-R-Ab assays were negative in all 174 controls. In contrast, the Turbo TSAb, Thyretain® TSAb and the binding assays detected TSAb in 557 of 670 (83%), 526/670 (78%) and 430/637 (67%) AITD samples, respectively (TurboTM bioassay vs binding immunoassay, chi-square test P < 0.001). The results of the Turbo bioassay highly correlated with both thyroid function in Graves’ hyperthyroidism as well as with the clinical activity and severity of Graves’ orbitopathy (P < 0.001). Inter- (two users) and intra- (two lots) assay comparison for the Turbo bioassay showed coefficient of variations of only 4%, 3%, and 2%, respectively. The two TSAb bioassays, TurboTM and Thyretain®, correlated (Spearman’s r =0.7, P < 0.001). Furthermore, the magnitude of serum levels in both TSAb bioassays correlated with TSH-R-Ab binding (both P < 0.001). Finally, seventeen AITD samples showed dual TSH-R-Ab positivity in the TBAb and TSAb bioassays.

Conclusions: The novel, rapid, easy-to-perform, sensitive and specific TurboTM bioassay performs better than the established TSH-R-Ab binding immunoassay and the Thyretain® bioassay. It provides clinically useful information for the accurate diagnosis and management of AITD patients.