Radio-receptor assay of TSH: its use to detect thyroid-stimulating immunoglobulins

Abstract

Immunoglobulins (IgG) in sera of patients with autoimmune thyroid diseases have been characterized by their ability to bind to thyroid TSH receptor sites. Thyroid membranes were prepared in hypotonic medium and discontinuous sucrose gradient flotation. 125l labelled-bovineTSH bound to the membranes at 0C for 120 min. Binding was linear with membrane concentration. Specific binding was maximum at pH 6.5, but binding studies with IgG were conducted at pH 7.5 in order to keep IgG soluble. Progressive inhibition of binding of label occurred at TSH concentration of 20 ng–50μg/ml, with half displacement at 7–70μg/ml. Unfractionated sera caused nonspecific depression of response. IgG was purified by DEAE-Sephadex absorption and used in concentration of 1.2 mg/ml or 175 μg/tube. Compared to the effect of a normal control pooled IgG on binding (“100%”), IgG from 17 normal subjects averaged 99±9% (range: 120–82%), and from 12 patients with thyroid cancer, 96±11%, both showing no inhibition of 125I-TSH binding. IgG from 15 patients with untreated Graves’ disease significantly (p<0.05) inhibited binding (81±17% of control) with up to 60% displacement of TSH. 53% of the patients were more than 2 SD below the mean normal value. Binding with IgG from sera of 8 patients with Hashimoto’s disease ranged from 61% to 120% of control. IgG from most patients with Graves’ disease compete with TSH for binding to thyroid cell membranes, and thus behave as antibodies to TSH receptors. Some patients with clinical thyroiditis have similar responses, confirming the hypothesis of a common immunologic background in these two diseases.