Clinical studies examining T monotherapy for the treatment of ED have yielded varying results. Meta-analysis, including trials in young organic hypogonadal patients, has shown a 57% response rate for T therapy in patients with ED, ranging from 64% for primary hypogonadism to 44% for secondary hypogonadism . Morales et al. showed that T therapy is an effective treatment for hypogonadal impotence, with improvement in sexual attitudes and performance in 61% of patients . In another study, T monotherapy has been observed to improve erectile function in only 36% of the hypogonadal patients consulting for ED  (see also pp. 203-2).
T therapy may have more significant effects on libido than on erectile function . In one study, normalization of serum T levels in hypogonadal men with ED resulted in only short-term improvement (one month) in erectile function and sexual satisfaction, while improvement of sexual desire was statistically significant for the six months of the study, making the use of T therapy alone questionable in this population . However, T monotherapy did improve sexual performance, desire, and motivation in men with hypogonadism, in clinical trials with transdermal T-gel formulation. Maximal improvement occurred on day 30 and continued for the six-month duration of the study .
There is a lack of data suggesting the efficacy of T therapy in older men who do not meet the clinical definition of hypogonadism. There is no convincing evidence that androgen therapy is either effective or safe for older men, unless a formal diagnosis of hy-pogonadism exists . The US Institute of Medicine (IOM) has recommended additional clinical research focusing on the benefits of T therapy in older men as compared with placebo controls, followed by larger-scale and longer-term trials to assess risks and benefits .
PDE5i are the first line of therapy in men who do not have potentially reversible causes of ED, such as hy-pogonadism . Nonetheless, 23% to 50% of patients do not respond to PDE5i alone [73,172]. Given the role of T in the NO pathway central to proper erectile function, interest in PDE5i-testosterone combination therapy has risen in recent years . As reported on p. 232, the most robust arguments supporting the validity of such a combination come from a randomized, placebo-controlled study of hy-pogonadal men with ED, non-responders to silde-nafil . In addition to improving erectile function,
T therapy improved orgasmic function . In another randomized, placebo-controlled study, short-term transdermal T administration improved the erectile response to sildenafil by increasing arterial inflow to the penis during sexual stimulation . T was also shown to improve arterial flow and subsequent response to tadalafil treatment, with a greater response after 10 weeks, compared to four weeks of pretreatment with T . Lastly, in a recent RCT, sildenafil improved sleep-related erections even in hypogonadal men, but combined therapy with T had greater effects than the sum of the two compounds alone, proving synergy .
Other uncontrolled studies have reported beneficial effects of combination therapy in patients with co-morbid conditions. Administration of intramuscular T and sildenafil was found to be efficacious in renal transplant patients and in patients on renal dialysis . Oral T has been reported to reverse ED associated with type 2 diabetes in patients failing on sildenafil therapy alone . In conclusion, T combination therapy with PDE5i may improve the response to PDEi in patients with ED and hypogonadism.
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