The purpose of this study is to examine the effects of metformin, testosterone, or both, in males with obesity-related hypogonadism. This study will evaluate changes in insulin resistance, weight loss, body composition, testosterone levels,quality of life and erectile dysfunction.

Obese men will receive diet and physical activity counselling, and be randomised to either testosterone, metformin, a combination of both or placebo

Inclusion Criteria:

• Men aged 18 to 50 years.
• BMI > or = 30 kg/m2.
• Total testosterone levels <2.3 ng/ml (<8 nmol/l) or testosterone levels 2.3-3.49 ng/ml (8-12 nmol/l) and free testosterone levels <70 pg/ml.
• LH levels <7.7 mIU / ml.
• No evidence of any other pituitary hormone disruption in morning blood sample (normal concentrations of TSH, FT4, Prolactin, ACTH, cortisol and IGF-1
• Being able to provide informed consent before randomization and agree to comply with all the procedures included in the protocol.

Exclusion Criteria:

• Intolerance/allergy to metformin or testosterone undecanoate.
• Previous diagnosis of diabetes mellitus (HbA1c> 6.5% or fasting glucose> 126 mg/dl or glucose> 200 mg/dl after an oral glucose tolerance test)
• Treatment with oral hypoglycemic agents, insulin or GLP-1 analogs.
• Poor kidney function: serum creatinine> 2.0 mg / dl.
• Previous history of prostate cancer or breast cancer.
• Active cancer of any kind.
• History of liver tumor or acute or chronic liver disease with impaired liver function: total bilirubin> 2.0 mg / dl or GOT levels three times the upper limit of normal.
• Central hypogonadism of organic cause
• Use in the past 12 months of any drug that affects the pituitary-gonadal axis.
• Use of oral testosterone, oral or transdermal within 2 weeks prior to study entry, or any testosterone ester in the last 6 weeks or testosterone undecanoate injection in the 6 months prior to study entry.
• Uncontrolled hypertension (SBP> 160 mmHg or DBP> 100 mmHg) despite adequate antihypertensive therapy.
• HIV infection or known active infection with HBV or HCV.
• Thrombotic or embolic disease.
• Heart disease, kidney or liver disease.
• Epilepsy or migraine not adequately controlled with treatment.
• Hematocrit> 50% in the screening.
• PSA> 4 ng / ml.
• Severe benign prostatic hypertrophy with an IPSS scale score over 19.
• Evidence of drug or alcohol abuse (> 50 g alcohol / day)
• Hematological diseases that produce increased risk of bleeding after intramuscular injection.
• Serious underlying disease that might affect the patient's ability to participate in the study (eg ongoing infection, gastric ulcers, active autoimmune disease).
• Reduced life expectancy (<12 months) by the presence of comorbidities or advanced terminals.
• Participation in another clinical trial within 30 days before study entry.
• Previous diagnosis of hemochromatosis
• Treatment with phosphodiesterase-5 inhibitors: sildenafil, tadalafil or vardenafil