OR18-03 Functional TSH Receptor Antibodies Are a Biomarker for Graves’ Disease - a Prospective Trial

Objective We aimed to evaluate the clinical utility and predictive value of stimulatory (TSAb) and blocking (TBAb) TSH receptor antibodies in the management of Graves’ disease (GD). Methods Hundred well-defined, consecutive, unselected, untreated hyperthyroid patients with GD were enrolled in a prospective two-year trial. Methimazole (MMI) was administered for 24 weeks according to baseline serum concentrations of free T3/free T4. Starting dose was 5–30 mg/day. Through a titration regimen, this dose was respectively tapered or increased at each subsequent study visit as the patient became euthyroid or remained hyperthyroid. Goals of therapy were to maintain normal fT4 and TSH levels. MMI therapy was stopped at week 24. The main outcome measure was clinical response versus non-response to a 24-week MMI treatment defined as biochemical euthyroidism versus persistent hyperthyroidism at week 24 and/or relapse at weeks 36, 48, and 96. TSAb was reported as percentage of specimen-to-reference ratio (cut-off SRR% <140). Blocking activity was defined as percent inhibition of luciferase expression relative to induction with bovine TSH alone (cut-off >40% inhibition). Results Forty-four patients responded to MMI of whom 43% had Graves’ orbitopathy (GO) while 56 were non-responders (66% with GO, p<0.01). At baseline, undiluted serum TSAb but not thyroid binding inhibiting immunoglobulins (TBII) differentiated between thyroidal GD only versus GD+GO (p<0.001). Further, at baseline responders demonstrated marked differences in diluted TSAb titers compared with non-responders (p<0.001). All patients with a TSAb dilution titer above three did not respond to MMI treatment. In contrast, TBII dilution titers did not differentiate between responders and non-responders to MMI and serum samples became TBII negative already at low dilutions. During treatment, serum TSAb levels decreased markedly in responders (p<0.001) but increased in non-responders (p<0.01). In contrast, TBII strongly decreased in non-responders (p=0.002). All non-responders at week 24 and/or those who relapsed during the 72-week follow-up were TSAb positive at week 24. A shift from TSAb to TBAb was noted in eight patients during treatment and/or follow-up and led to remission. Conclusions Serum TSAb levels are a biomarker for and mirror severity of GD. Their increase during MMI treatment is a marker for on-going disease activity. TSAb dilution analysis had additional predictive value.