Testosterone replacement therapy (TRT) is indicated for the treatment of symptomatic hypogonadism. TRT increases libido, improves erectile quality and builds lean muscle mass, and bone mineralization whilst reducing body fat. A suspected diagnosis of low testosterone is confirmed with two morning fasting total testosterone levels taken between 07.00 and 11.00. Patients with levels greater than 12 nmol/l (3.5 ng/ml) are unlikely to benefit from TRT whilst those below 8 nmol/l and those between 8 and 12 nmol/l suffering significant symptoms are most likely to benefit. Where clinical, and biochemical doubt, remains a free testosterone of <220 pmol/l is indicative of a more positive response to TRT.