Only two mutations at the Lysine 183 amino acid in the extracellular N-terminal domain of human TSH receptor (hTSHR) have been associated to hypersensitivity to hCG and familial gestational hyperthyroidism.A 38-year-old woman was seen during the first trimester of her second pregnancy for thyrotoxicosis with increased fT3 and fT4 concentrations and low TSH levels without anti-TSH receptor antibody. Thyrotoxicosis improved spontaneously during the 2nd trimester and persisted at the 3rd trimester. Similar clinical symptoms (weight loss, nausea, vomiting) were also reported during the first trimester of her first pregnancy and the first pregnancy of her mother.DNA sequencing of the hTSHR gene of this woman and her mother, identifies a heterozygous variant changing Valine to Isoleucine residue at codon 597 in the transmembrane domain (TMD) of this receptor. In vitro functional studies of this variant showed increased constitutive activity in regard to the basal level of cAMP and IP3 production and to the low cell surface expression, while response to TSH was reduced compared to that of the wild type receptor. The Val597Ile variant presented a dose-dependent increase in cAMP response to hGC and human luteinizing hormone (hLH). Simulation of the protein dynamics showed a high structural impact of the Val597Ile variant on helices 3 (TMH3) and 5 (TMH5) of the transmembrane domain participating to constitutive activity and hCG sensitivity.We describe a new variant in the transmembrane region of the hTSHR gene with increased constitutive activity and hCG hypersensitivity in familial gestational hyperthyroidism.