Radioreceptor assay of anti-TSH receptor antibody activity: comparison of assays using unextracted serum and immunoglobulin fractions, and standardization of expression of activities.

A radioreceptor assay for TSH receptor antibodies developed by Shewring and Smith is described in which unextracted serum is used. The assay is simple and reproducible. TSH-binding inhibitor immunoglobulin (TBII) activity determined using unextracted serum correlated well with that determined using the immunoglobulin fraction purified with polyethylene glycol. The assay detected TSH receptor antibodies in 81 of 85 patients (95%) with untreated Graves' disease, no patients with Graves' disease in remission, and 7 of 57 patients (12%) with Hashimoto's disease. TSH-binding inhibitor activity of a human TSH preparation (Kabi Diagnostica) was not detectable at 100 mU/L, but detectable at 1,000 mU/L. Consistent with this, TBII activity was not detectable in hypothyroid patients with goitrous Hashimoto's disease, whose serum TSH concentrations were 11-520 mU/L. Of 57 patients with Hashimoto's disease positive TBII activity was detected in only 7 of 15 patients (46%) with primary atrophic hypothyroidism. Five of these had very high TBII activities (greater than 90% inhibition of labelled TSH binding), and no linear dose-response relationship was observed in dilution experiments. For exact determination of TBII activity in specimens with high activities, serum samples were diluted with normal pooled serum and activities were expressed in U/ml by comparison with values for standard serum, prepared from bovine TSH diluted with normal pooled serum. In this way, samples with TBII activities of 1-50 U/ml (20-90% inhibition of binding of labelled TSH) showed linear dose-response relations in dilution experiments, and the dilution curves of all samples examined were parallel to the standard curve. This standardization procedure was used in determining the half life of TBII.