Although there are no standard criteria for the diagnosis of latent hyperprolactinemia using recent assay systems, we diagnosed latent hyperprolactinemia based on the observation that serum PRL increased to more than 140 ng/mL after the TRH provocation test, serum PRL was increased more than 20-fold by TRH, and that our patient had symptoms associated with hyperprolactinemia. This evidence concerns the gene TRH and hyperprolactinemia.