It has recently been suggested that the low-density lipoprotein receptor-related protein 5 (LRP5) regulates bone mass by suppressing secretion of serotonin from duodenal enterochromaffin cells. In mice with targeted expression of a high bone mass–causing (HBM-causing) LRP5 mutation and in humans with HBM LRP5 mutations, circulating serotonin levels have been reported to be lower than in controls whereas individuals with loss-of-function mutations in LRP5 have high blood serotonin. In contrast, others have reported that conditionally activating a knock-in allele of an HBM-causing LRP5 mutation in several tissues, or genetic deletion of LRP5 in mice has no effect on serum serotonin levels. To further explore the possible association between HBM-causing LRP5 mutations and circulating serotonin, levels of the hormone were measured in the platelet poor plasma (PPP), serum, and platelet pellet (PP) of 16 affected individuals from 2 kindreds with HBM-causing LRP5 mutations (G171V and N198S) and 16 age-matched controls. When analyzed by HPLC, there were no differences in levels of serotonin in PPP and PP between affected individuals and age-matched controls. Similarly, when analyzed by ELISA, there were no differences in PPP or PP between these two groups. By ELISA, serum levels of serotonin were higher in the affected individuals when compared to age-matched controls. A subgroup analysis of only the G171V subjects (n = 14) demonstrated that there were no differences in PPP and PP serotonin between affected individuals and controls when analyzed by HPLC. PP serotonin was lower in the affected individuals when measured by ELISA but serum serotonin levels were not different. We conclude that there is no change in PPP serotonin in individuals with HBM-causing mutations in LRP5. © 2014 American Society for Bone and Mineral Research.