Résumé : An infant girl was born at 37 weeks gestation and found to be clinically thyrotoxic at 9 months of age. Thyroid autoantibodies were negative, and thyroid function failed to normalize with medical treatment. The patient underwent a total thyroidectomy. DNA obtained from her thyroid gland and leukocytes was analyzed for thyrotropin receptor (TSHR) mutations using single strand conformation polymorphism and direct sequencing. A mobility shift of polymerase chain reaction (PCR)-amplified DNA was detected on single strand conformation polymorphism gel. Direct sequencing identified a novel point mutation in the fifth transmembrane domain of the TSH receptor at codon 597 (GTC to CTC), resulting in the amino acid substitution of leucine for valine. The mutation was heterozygous and germline, and was not identified in DNA from either of her parents. Expression of the V597L mutant is transiently transfected COS 7 cells displayed increased constitutive cyclic adenosine monophosphate (cAMP) production compared with the wild-type receptor. The mutant is expressed at very low levels on the surface of COS cells, and its response to TSH is marginal.