TSHR autoantibodies with stimulating (agonist) activity cause the hyperthyroidism of Graves disease, while the rarer blocking (antagonist) type TSHR autoantibodies are responsible for hypothyroidism in some patients. We have isolated four human monoclonal autoantibodies (MAbs) which bind the TSHR with high affinity. Two antibodies, M22 and K1-18 have potent thyroid stimulating activity, one MAb K1-70 has strong antagonist (blocking) activity and the fourth MAb 5C9 has strong antagonist and inverse agonist activity. Stable complexes of the TSH receptor (amino acids 1260; TSHR260) bound to the blocking type human MAb K1-70 and to the stimulating MAb M22 were prepared and analysed by X-ray diffraction. The structure of the TSHR260 in complex with K1-70 Fab (solved at 1.9 Æ resolution) is virtually identical to the structure of the TSHR260 in complex with the thyroid stimulating MAb M22 previously solved at 2.55 Æ resolution. The M22 (agonist) positions itself on the TSHR in a similar orientation as TSH (agonist) with the M22 heavy chain mimicking the interactions of the TSH α chain and the M22 light chain mimicking the interactions of the TSH β chain. In contrast, the K1-70 MAb binds more N-terminally on the concave surface of the TSHR interacting extensively between amino acids D36 and D203, whilst the binding site of M22 extends further towards the C terminus and covers TSHR amino acids D36 to N256. These differences may reflect the distinct functional activities of M22 and K1-70. However, the binding sites of M22, TSH and K1-70 overlap extensively with 12 TSHR amino acids interacting with all 3 ligands. The availability of high resolution crystal structures of the TSHR in complex with a blocking monoclonal autoantibody (K1-70) and stimulating monoclonal autoantibody (M22) provides a foundation for developing new strategies to understand and control TSH receptor activation and the autoimmune response to the TSHR.