no species specificity of TSH receptor antibodies was detected for patients with GD; a selective species specificity for human and rat TSH receptors was found for HT sera. This suggests that TSH receptor antibodies in HT either recognize different determinants on the receptor than the antibodies in GD or are of lower affinity. Furthermore, the use of porcine thyroid tissue or GPFC may lead to an underestimation of the presence and level of TSH receptor antibodies in patients with HT.