A mutated TSHR preparation which includes at least one point mutation characterized in that at least amino acid Arg at a position corresponding to amino acid 255 of a full length human TSHR has been mutated to a different amino acid residue in said mutated TSHR preparation, whereby said mutated TSHR preparation differentially interacts with patient serum stimulating TSHR autoantibodies, patient serum blocking TSHR autoantibodies and TSH, in that (i) the stimulatory effect of patient serum stimulating TSHR autoantibodies interacting with the mutated TSHR preparation is substantially reduced or essentially abolished, when compared to the stimulatory effect of the patient serum stimulating TSHR autoantibodies interacting with a reference TSHR preparation which has an amino acid sequence corresponding to that of said mutated TSHR preparation with the exception that said mutation of Arg at a position corresponding to amino acid 255 of a full length human TSHR is not present in said reference TSHR preparation, (ii) the stimulatory effect of TSH when interacting with the mutated TSHR preparation is essentially unaffected, when compared to the stimulatory effect of TSH interacting with said reference TSHR preparation, and (iii) the blocking effect of patient serum blocking TSHR autoantibodies interacting with the mutated TSHR preparation is essentially unaffected or increased, when compared to the blocking effect of the patient serum blocking TSHR autoantibodies interacting with said reference TSHR preparation, whereby said mutated TSHR preparation is effective in the differential screening and identification of patient serum stimulating TSHR autoantibodies, patient serum blocking TSHR autoantibodies and TSH in a sample of body fluid being screened.
