FIG. 2.

Difficulty in measuring TSH-blocking autoantibodies (TBAb) in the presence of thyroid-stimulating autoantibodies (TSAb). (A) Schematic depiction of a range of agonists of increasing potency. The TSHR, similar to many receptors, has a degree of ligand-independent or constitutive activity (27,28), as shown by the horizontal line through the wedge. TSHR ligands (TSH or TSAb) further increase receptor activity (not shown to scale). An inverse agonist suppresses constitutive activity, whereas a full agonist maximally activates the receptor. Ligands of intermediate activity are either neutral agonists (no activation of the receptor or suppression of constitutive activity) or partial agonists or inverse agonists. Ligands can also be an antagonist for another ligand depending on their relative affinities and binding sites. Therefore, a TSAb that is a partial (not a full) agonist can also be an antagonist for TSH. If a serum displays both TSAb and TBAb activity, unless the former is very weak and the latter is very strong, it cannot be assumed that there are two separate antibodies. (B) Difficulty in quantifying TBAb activity in the presence of TSAb. All TBAb bioassay data are expressed as the percent decrease in TSHR activation induced by TBAb relative to a baseline denominator, the latter being the activity of a standard, constant, and low TSH concentration. However, if TSAb activity is also present, different reports in the literature either do, or do not, subtract this TSAb value from the baseline denominator. This variation can have a major effect on deciding whether a TBAb is positive. For example, in a serum lacking TSAb activity (i), a patient's IgG or serum may inhibit TSH activity by 30%. Since many reports require 30%–40% inhibition of TSH activity to establish TBAb positivity, this serum would be regarded as TBAb negative. However, if weak intrinsic TSAb activity (ii) is present in the same serum and is subtracted to establish the “100%” TSH denominator, a 30% suppression of TSH activity represents a calculated TBAb activity of 50%, which is now positive. In an extreme example (iii), a stronger TSAb serum occupying most TSHR on the cell surface is a partial agonist, generating a 70% signal of that for TSH. This serum suppresses TSH activity by the same 30% but has a calculated TBAb of 100% after TSAb subtraction despite the absence of TBAb. In our view, therefore, it is preferable not to subtract TSAb activity in calculating the TBAb assay data.