Discrepancy between Thyroid-Stimulating and Thyrotropin-Binding Inhibitory Activities of Graves' Immunoglobulin Gs Assessed in the Mouse*

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Abstract
TSH receptor binding-inhibiting (TBI) and thyroid-stimulating (TSI) activities of Graves'' immunoglobulin G (IgG) were measured using the murine TSH receptor (mTBI) and the McKenzie LATS [long-acting thyroid stimulator] bioassay. In contrast to the expectation that only the IgG positive for LATS showed mTBI activity, mTBI activity was closely correlated with TBI activity measured at the human TSH receptor regardless of LATS potency (r = 0.79; P < 0.01), indicating a lack of species specificity of IgG in this particular aspect. In 7 of 15 IgG positive for both mTBI and LATS, there was a significant correlation between the 2 parameters (r = -0.89; P < 0.01). However, 4 IgG among the 15 were positive for mTBI but negative for LATS even when tested at 3- to 7-fold concentrations of IgG. There was 1 IgG negative for mTBI but definitely positive for LATS activity. Thus, the discrepancy between TBI and TSI activity of the Graves'' IgG was manifested at the murine TSH-receptor, suggesting heterogeneity of anti-TSH receptor antibody.

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