Hypogonadism and erectile dysfunction: the role for testosterone therapy.
The role of low testosterone levels in erectile dysfunction (ED) remains unclear. Both organic and psychogenic factors contribute to ED, with vasculogenic causes being the most common etiology. Approximately 10-20% of patients with ED are diagnosed with hormonal abnormalities. At the physiologic level, two second messenger systems are involved in mediating erections, one involving cyclic adenosine monophosphate (cAMP) and the other involving cyclic guanosine monophosphate (cGMP). PDE5 inhibitors such as sildenafil promote the cGMP pathway, while alprostadil affects the cAMP pathway. Evidence is strong that, in animal systems, testosterone has direct effects on erectile tissue. However, although testosterone clearly has an impact on libido in humans, its effect on penile function is less clear. Evaluation of ED includes medical, sexual, and psychosocial history assessments, as well as laboratory tests to check for diabetes and hormonal abnormalities. Initial interventions should involve correction of potentially reversible causes of ED, such as hypogonadism. First-line therapy for other patients is typically oral PDE5 inhibitors, such as sildenafil, tadalafil, or vardenafil. For patients who fail treatment with PDE5 inhibitors, local therapies such as intracavernous alprostadil are highly successful. Recent data also support the success of combination therapy with sildenafil and testosterone. This opens the possibility of other combinations of testosterone and other treatments of ED. The ability to exploit multiple pathways in the physiologic processes leading to erection may help improve therapy for ED.