ObjectiveThere is a bidirectional interaction between circulating testosterone and blood glucose levels. We aim to investigate the testosterone levels in men with early-onset type 2 diabetes (T2DM).MethodsA total of 153 drug naive men with T2DM were enrolled in the study. Early- (n= 63) and late-onset (n= 90) T2DM was classified according to age 40 years old. Clinical characteristics and plasma for biochemical criterions were collected. Gonadal hormones were measured using chemiluminescent immunometric assay. The concentrations of 3β- and 17β-HSD were determined using ELISA.ResultsCompared with men with late-onset T2DM, those with early-onset T2DM had lower serum total testosterone (TT), sex hormone-binding globulin (SHBG), and FSH, but higher dehydroepiandrosterone sulfate (DHEA-S) level (p< 0.05). The mediating effect analysis showed that the decreased TT levels in patients with early-onset T2DM were associated with the higher HbA1c, BMI, and triglyceride in these patients (bothp< 0.05). The early-onset of T2DM directly correlated with increased DHEA-S (bothp< 0.01). The 3β-HSD concentration in the early-onset T2DM group was lower than that in the late-onset T2DM group (11.07 ± 3.05 vs. 12.40 ± 2.72 pg/mL,p= 0.048) and was positively correlated with fasting C-peptide, while negatively correlated with HbA1c and fasting glucagon (pall < 0.05).ConclusionsPatients with early-onset T2DM showed inhibition of conversion from DHEA to testosterone, which may attribute to the low level of 3β-HSD and high blood glucose in these patients.