Testosterone and erection in normal men

So far, only small-scale studies have been performed testing the effects of testosterone on erection in normal men. In addition, these studies do not allow exact differentiation between effects on sexual behaviour and direct effects on the penis.

Time (days)

Fig. 11.1 Effects of treatment of hypogonadal men (n = 227) with testosterone gel (squares, 50 mg/d;

circles, 100 mg/d) and non-scrotal testosterone patches (triangles, 5 mg/d) on erection as assessed by a questionnaire on percentage of full erection (left panel) and satisfaction with erection (right panel) (modified with permission from Wang et al. 2000, copyright 2000, The Endocrine Society).

One controlled study involving 11 normal men tested the effects of varying serum testosterone concentrations within the normal range (Buena et al. 1993). All men received the depot GnRH agonist leuprolide acetate for suppression of endogenous testosterone to the hypogonadal range. Six volunteers received 4 mg/d of a testosterone microcapsule formulation to restore testosterone levels to the low normal range (mean values 10.5 ± 1.7 nmol/l), whereas five volunteers received a dose of 8 mg/d resulting in testosterone levels in the middle to high normal range (mean values 26.5 ± 3.4 nmol/l). Despite significantly different testosterone levels, albeit in the normal range, there was no difference in the number of spontaneous nocturnal erections during rapid eye movement (REM) periods as well as no difference in the magnitude and duration of tumescence as measured by NPT recordings at the base and the tip of the penis.

When ten healthy, young adult males received the depot GnRH agonist leuprolide acetate or placebo for 12 weeks, but remained without androgen substitution, the suppression of endogenous testosterone to the low hypogonadal range resulted in a significant decrease of sleep-related erections (Hirshkowitz etal. 1997). Whereas sleep efficiency and REM sleep measures did not differ between groups, the total tumescence time of sleep-related erections at the penile base decreased significantly compared to placebo. The observed reduction of maximal circumference increase and frequency of erections did not reach statistical significance.

Comparable results were seen in studies concentrating on sexual behaviour (see Chapter 4). In short, suppression of endogenous testosterone by the GnRH antagonist Nal-Glu in normal men and substitution of exogenous testosterone with low (50 mg/week) and high doses (100 mg/week) of testosterone enanthate did not change the frequency of sexual desire, sexual fantasies, intercourse, or spontaneous erections. Addition of the aromatase inhibitor testolactone had no effect on sexual behaviour, whereas significant effects on sexual behaviour were noted in those men receiving the GnRH antagonist and placebo (Bagatell etal. 1994).

Weekly administration of 25, 50,125, 300 and 600 mg of testosterone enanthate for 20 weeks to eugonadal men who had received a GnRH agonist for suppression of endogenous testosterone did not change scores for sexual activity and sexual desire (Bhasin et al. 2001), whereas other androgen-dependent parameters, such as fat-free mass, muscle size, strength and power, haemoglobin, HDL cholesterol and IGF 1, showed significant dose-response relationships. Similarly, administration of high doses of weekly 200 mg testosterone enanthate for eight weeks in a single-blind, placebo-controlled study in normal men did not induce changes in parameters of sexual activity (Anderson etal. 1992). One early placebo-controlled study in eight normal eugonadal men found enhanced rigidity of nocturnal penile tumescence after administration of 150 mg testosterone enanthate, but no effect on frequency of erections or circumference increase of the penis (Carani et al. 1990).

These experimental studies in normal men indicate that variations of testosterone levels within the normal range or serum levels exceeding the upper limit of normal have no or very limited influence on erectile function. This conclusion is in agreement with the results of two larger studies correlating serum levels of testosterone with erectile function in normal men. In one study involving 201 men, serum levels of testosterone in the normal range did not show any significant association with parameters of nocturnal penile tumescence and rigidity monitoring, whereas men with serum levels lower than 200 ng/dl had significantly lower values of the respective erectile parameters (Granata et al. 1997). In a recent large study involving 1071 mainly eugonadal men aged from 40 to 90 years, no significant association was detected between serum testosterone levels and the prevalence or severity of erectile dysfunction as assessed by the questionnaire of the simplified International Index of Erectile Function (IIEF-5) (Rhoden etal. 2002).

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