Serial dilution curves of international standard material of thyroid-stimulating immunoglobulins

Serial dilution curves of international standard material (IS-90/672) of thyroid-stimulating immunoglobulins (TSIs). The two curves are parallel to each other along their linear portions, indicating that the two tests have similar responses to IS-90/672. The TSI bioassay curve is shifted to the right, suggesting better detection sensitivity than the thyrotropin receptor autoantibody-binding (TRAB) assay.

At very high TSI concentrations, the TSI bioassay might be less accurate than the TRAB assay because of a high-dose hook effect. The reference range for the TSI bioassay is a TSI index of <1.3; the reference range for the TRAB assay is <16 percent thyrotropin (TSH)-binding inhibition.