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 gα-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°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.