To investigate renal function in idiopathic hypogonadotropic hypogonadic (IHH) patients by measuring glomerular filtration rate (GFR) using modification of diet in renal disease formula, and determine whether there is any relationship between GFR and testosterone levels. Thirty-three patients with IHH and 37 healthy control subjects participated in this study. The IHH group showed statistically significant higher GFR and proteinuria with respect to the control group (163.1 +/- A 46.9 to 117.9 +/- A 30.5 mL/min, p < 0.001; 0.2 +/- A 0.1 to 0.08 +/- A 0.02 mg/dL, p = 0.041, respectively). Uric acid and creatinine levels were statistically lower than in the control group (4.6 +/- A 0.5-3.6 +/- A 0.9 mg/dL, p = 0.02; 0.7 +/- A 0.2 to 0.9 +/- A 0.2 mg/dL, p < 0.001, respectively). Hyperfiltration positively correlated with IHH in multivariate linear regression analyses (beta = 0.591, p < 0.001). In addition, in the IHH group, we found that the GFR increased independently of body mass index and age. Our study confirms that low testosterone in IHH patients is associated with glomerular hyperfiltration. Patients with IHH should be carefully monitored with respect to their GFR.