Comparisons of different assays for the thyroid-stimulating antibody of Graves' disease

Various current methods for the assay of the thyroid-stimulating antibody of Graves' disease (TSAb) were assessed for comparative sensitivity and specificity. Five procedures were examined in detail: I) increase in cAMP in human thyroid slices; II) inhibition of [125I]TSH binding to human thyroid membranes, particulate or III) solubilized; IV) inhibition of [125I]TSH binding to guinea pig fat cell membranes, particulate or V) solubilized. The minimum effective concentration of TSH (microunits per ml) in these assays was: I) 5; IV) 12.5; V) 25 and II and III) 100. The guinea pig fat systems (IV and V) were more sensitive than the human thyroid assays (II and III) in terms of the concentration of TSH required for 50% inhibition of binding. Five preparations of TSAb-immunoglobulin G were found in general to have constant relative potency in the five assay systems, but individual immunoglobulins G were variable in their effects when data in the five procedures were compared directly. It is probable that, in addition to TSAb, other antibodies, more or less specific for the human thyroid, influenced the TSH binding inhibition assays to a variable extent. The above TSH binding inhibition assays were carried out with solutions or suspensions of receptor preparations. Solid phase systems, using microtiter plates for TBI assays, were assessed in less detail. Human thyroid, particulate or solubilized, was ineffective (little specific binding of [125I]TSH), but a particulate preparation of guinea pig fat cell membranes was an efficient basis for a solid phase TBI system. Solid phase assessment with [125I]staphylococcal protein A of binding of thyroid autoantibodies to human thyroid membranes was not specific for TSAb.