Anti-bovine thyrotropin autoantibodies in patients with Hashimoto's thyroiditis, subacute thyroiditis, and systemic lupus erythematosus

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.

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

A previous report demonstrated that sera with bovine TSH (bTSH) binding antibody showed abnormally negative TSH receptor antibody (TRAb) activity in the standard TRAb assay method. The corrected TRAb activity calculated by the determination of the nonspecific binding of the labeled bTSH for each test serum [NSB(T)] resulted in positive TRAb activity. However, the precise calculation was difficult because NSB(T) level was significantly higher than the nonspecific binding in normal pool serum [NSB(N)] level. In the present experiment the determination of the TRAb activity was performed after absorption of bTSH binding antibodies by the heat-denatured bTSH to obtain more precise TRAb activity. In addition, absorption by sheep FSH (sFSH) was performed because almost all bTSH binding antibodies showed specific binding to the alpha-subunit of mammalian pituitary glycoprotein hormones in our previous study. Three days absorption of test serum using 1 mU of the heat-denatured bTSH (100 degrees C for 1 h) or 5 mU of sFSH was chosen as optimal because NSB(T) decreased remarkably to NSB(N) levels. The corrected TRAb determined after these absorptions decreased significantly compared to the corrected TRAb activity without the absorption. When the complete absorption of bTSH binding antibody was performed by the decrease of the NSB(T) level to the NSB(N) level, the TRAb activity determined by these two different absorptions was almost similar (difference was less than 10%). However, it was difficult to obtain the precise TRAb activity in the cases with extremely high bTSH binding antibody, because the NSB(T) level was higher than the NSB(N) level by the incomplete absorption of bTSH binding antibody.

Evaluation of thyroid function in patients with thyroid hormone autoantibodies

Serum free thyroxine (FT4) levels were measured in patients with Hashimoto's thyroiditis and Graves' disease who were seropositive for thyroid hormone autoantibodies. The methods used were equilibrium dialysis/radioimmunoassay (FT4 by equilibrium dialysis, Nichols Institute) and FT4 analog radioimmunoassay (Amerlex MFT4) before and after treatment of sera with 12.5% polyethylene glycol (PEG). Furthermore, FT4 measurement in the PEG-treated sera was done using two other analog radioimmunoassays (DPC FT4 Kit and N-FT4 Corning). Serum thyrotropin (TSH) concentration before and after i.v. infusion of thyrotropin releasing hormone (TRH) was measured in five cases of Hashimoto's thyroiditis and four cases of Graves' disease with thyroid hormone autoantibodies. Although FT4 determination by analog tracer radioimmunoassays showed unusually high values for the hormone, results obtained by an equilibrium dialysis/radioimmunoassay method showed compatible values with basal TSH. Furthermore, FT4 concentrations measured with Amerlex MFT4 after treatment of sera with 12.5% PEG, correlated well with the values obtained by equilibrium dialysis/radioimmunoassay (r = 0.98, P < 0.001). Similar results were obtained with two other analog FT4 radioimmunoassays after treatment of sera with PEG. These results indicate that real FT4 values in patients with thyroid hormone autoantibodies are compatible with basal TSH concentrations. The presence of thyroid hormone autoantibodies per se does not affect the control mechanism of the hypothalamo-pituitary-thyroid axis. It was concluded that measurement of basal TSH as well as FT4 values by either equilibrium dialysis/radioimmunoassay or analog radioimmunoassays after PEG treatment provides clinicians with valid information for assessment of the precise status of thyroid function in patients with thyroid hormone autoantibodies.