A mutant thyroid stimulating hormone receptor (TSHR) or fragment thereof comprises one or more mutations, wherein the mutant TSHR has increased thermostability with respect to the equivalent wild type TSHR or fragment. The one or more mutation is preferably within the extracellular leucine-rich repeat domain (LRD) of the TSHR, or within residues 22 to 260 (TSHR260) of the TSHR, or may be in the transmembrane domain (TMD), A mutant TSHR or fragment thereof of the invention preferably consists of, or consists essentially of, the subdomain TSHR260 of the TSHR receptor. A mutant TSHR or fragment thereof according to the invention has a greater thermostability than the equivalent wild type TSHR or fragment as determined by its half-life at a given temperature, and can be purified whilst retaining activity. A mutant TSHR or fragment thereof according to the invention may also be deglycosylated whilst retaining activity. Methods, kits and uses employing the mutant TSHR or fragment thereof according to the invention are also provided.