Discrepancy between thyroid-stimulating and thyrotropin-binding inhibitory activities of Graves' immunoglobulin Gs assessed in the mouse.

TSH receptor binding-inhibiting (TBI) and thyroid-stimulating (TSI) activities of Graves' immunoglobulin Gs (IgGs) were measured using the murine TSH receptor (mTBI) and the McKenzie LATS bioassay. In contrast to the expectation that only the IgGs 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 less than 0.01), indicating a lack of species specificity of IgG in this particular aspect. In 7 to 15 IgGs positive for both mTBI and LATS, there was a significant correlation between the 2 parameters (r = -0.89; P less than 0.01). However, 4 IgGs among the 15 were positive for mTBI but negative for LATS even when tested at 3- to 7-fold concentrations of IgG.