Serum free testosterone and metabolic syndrome in Japanese men.

To examine the association between late-onset hypogonadism (LOH) and metabolic syndrome (Mets) or insulin resistance in the Japanese adult male population, we evaluated anthropometric parameters, indices of glucose and lipid metabolism, and hormones related to sexual function in 274 men (mean age: 46.0 +/- 11 years) who underwent general health checks. Seventy subjects (25.5%) were diagnosed as having Mets, while the frequency of LOH was 8.0%. Glycated hemoglobin was normal in the majority of participants (94.9%). The serum free testosterone (FT) level was significantly lower in the Mets (+) group than in the Mets (-) group (11.7 +/- 4.0 vs. 14.7 +/- 4.6 pg/mL, p<0.0001). FT decreased significantly along with an increase in the number of Mets components. Likewise, the number of Mets components showed a significant difference among the eugonadal, borderline, and hypogonadal groups (2.2 +/- 1.4, 1.5 +/- 1.4, and 0.9 +/- 1.1, respectively). After adjustment for age, body mass index (BMI), and waist circumference (WC), FT was still significantly correlated with Mets (standard partial regression coefficient = - 0.0971; 95 % confidence interval = - 0.1936 approximately - 0.0006; p = 0.048). A compensatory increase of gonadotropins was not seen in the hypogonadal group. Among Japanese men who were mainly without diabetes, FT was associated with Mets independently of age, BMI, and WC. Mets and insulin resistance may decrease serum testosterone via induction of hypogonadotrophic hypogonadism, and the reduction of testosterone may in turn cause further obesity and insulin resistance, consequently initiating a vicious cycle.