Serotonin Rising

To the Editor: Rosen's Perspective article (March 5 issue) 1 highlights recent findings that gut-derived serotonin inhibits bone formation by stimulating serotonin receptors on the preosteoblast. 2 A critical question is whether serotonin is delivered to bone in some blood element or as free plasma serotonin. The serum serotonin measurements used by Yadav and colleagues 2 reflect an undefined proportion of the platelet pool and say nothing about the minuscule and often mismeasured free plasma concentrations. 3 If the platelet is the delivery vehicle, it is paradoxical that increased platelet serotonin levels in Lrp5-knockout animals and patients with osteoporosis pseudoglioma lead to bone loss, . . .