Quantification of serotonin and eight of its metabolites in plasma of healthy volunteers by mass spectrometry

•We developed a mass spectrometry method for plasma serotonin and 8 metabolites.•The method was fully validated.•Sensitivity is sufficient to measure diurnal melatonin (LLOQ at 1.7 pM = 0.4 pg/mL)•Plasma concentrations for these analytes were determined in 102 healthy volunteers.•This method will enable drawing a detailed picture of the complex pineal metabolism.