Neonatal hyperthyroidism is a rare disorder and occurs in two forms. An autoimmune form is associated with maternal Graves' disease, resulting from transplacental passage of maternal thyroid-stimulating antibodies and a nonautoimmune form is caused by gain of function mutations in the thyrotropin receptor (TSHR) gene. Thyrotoxicosis caused by germline mutations in the TSHR gene may lead to a variety of clinical consequences. To date, 55 activating mutations of the TSHR gene have been documented. Fourteen cases with sporadic activating TSHR germline mutations have been described. Here we report a male infant with nonautoimmune hyperthyroidism due to an activating germline TSHR mutation (A623V), whose clinical picture started in the newborn period with severe hyperthyroidism. His parents did not have the same mutation. This mutation had been previously detected as a somatic mutation in patients with toxic adenomas. This is the first report of a sporadic case of nonautoimmune congenital hyperthyroidism associated with A623V mutation. © Journal of Clinical Research in Pediatric Endocrinology.