Constitutively activating thyrotropin receptor (TSHR) germline mutations have been identified as a molecular cause of congenital hyperthyroidism. We here describe a Japanese woman who had presented with severe hyperthyroidism and advanced bone age as a neonate. She underwent neurosurgical intervention for craniosynostosis, and presented with perodactylia and mild mental retardation with hydrocephalus. Hyperthyroidism has been refractory to antithyroid drug therapy in the absence of antithyrotropin receptor antibodies during follow-up of 20 years, resulting in an enlarged goiter. Analysis of the patient's genomic DNA showed a heterozygous thymine-to-adenine point mutation in exon 10 of TSHR at position 1535 which was not present in the parents' DNA. This mutation, changing leucine to glutamine in codon 512 in the third transmembrane region, was previously identified as a somatic mutation in toxic thyroid nodules and was shown to increase basal cAMP production in vitro. To our knowledge, this is the first report of a germline mutation of TSHR causing sporadic congenital nonautoimmune hyperthyroidism in a Japanese patient.