INSL3 and Klinefelter syndrome: Based on an established state-of-the-art liquid chromatography-tandem mass spectrometry (LC-MS/MS) technique for INSL3 quantification with associated male reference ranges (8, 9), we recently showed a dichotomy with lower INSL3 concentrations and higher testosterone concentrations in patients with hypogonadotropic hypogonadism and Klinefelter syndrome (10).