Precise determination of TSH receptor antibody activity in serum containing bovine TSH (bTSH) binding antibody by absorption using denatured bTSH or sheep FSH

Demonstration of anti-TSH antibody in TSH binding inhibitory immunoglobulin-positive sera of patients with Graves' disease

OBJECTIVES

The presence of anti-TSH antibodies in Graves' patients with unusually low TSH binding inhibitory immunoglobulin (TBII) has been reported. Recently, we found the first case of an anti-TSH antibody in TBII-positive sera of patients with Graves' disease. The prevalence and immunological specificity of this anti-TSH antibody were examined.

DESIGN

The presence of 125I-bovine(b) TSH binding antibody in TBII positive serum was examined by prolonged incubation of more than 1 day because only weak binding occurred after 1 h incubation at 37 degrees C. The clinical course of these patients and binding characteristics of anti-bTSH antibody were examined.

RESULTS

The corrected method-TBII activity (%)[1 - (a - b)/(c - d)] x 100 and the standard method-TBII activity (%) [1 - (a - d)/(c - d)] x 100 [a, 125I-bTSH binding with TSH receptor (R) in the presence of test serum; b, 125I-bTSH binding with test serum; c, 125I-bTSH binding with TSH R in the presence of normal serum; d, 125I-bTSH binding with normal serum] were calculated. The corrected method-TBII activity was always higher than the standard method-TBII activity in anti-bTSH antibody-positive serum. Anti-bTSH antibody-positive cases in TBII-positive Graves' disease were found in approximately 1% of Graves' patients. Anti-bTSH antibodies were confirmed as IgG from the increase of precipitated radioactivity by adding rabbit antihuman IgG antibody after the incubation of 125I-bTSH with test serum. These antibodies bind with not only bTSH, bTSH(alpha) and bLH, but also porcine (p)TSH, pTSH(alpha) and pFSH. However, these antibodies did not bind with human TSH. Binding of 125I-bTSH with patient's serum was neither inhibited by other Graves' thyroid stimulating antibody (TSAb), nor thyroid blocking antibody (TBAb) in primary hypothyroidism.

CONCLUSIONS

The presence of anti-bTSH antibody in TBII-positive serum of high titre means that TBII-positive sera cannot rule out the absence of anti-bTSH. Thus, determination of 125I-bTSH binding with test serum in TSH receptor assays is necessary to determine the precise TBII activity and to detect anti-bTSH antibody.