Influence of testosterone on sexual behaviour in men

The physiological range of testosterone levels (3-12 ng/ml) is considerably higher than that necessary to maintain normal sexual functions. Testosterone levels found to be critical for sexual functions in males lie around 3ng/ml (Nieschlag 1979), and they show a clear intersubject variation. On the other hand, levels at which a decline of androgen-related sexual behaviour in individual subjects occurs appears to be reproducible (Gooren 1987).

Table 4.1 Significantly positive effects of androgens (testosterone, DHT) on various aspects of sexual behaviour in men

Behaviour

Endogenous testosterone/ DHT level

Testosterone substitution

Sexual interest and

Nilsson etal. 1995

Anderson etal. 1992

phantasies

Bancroft 1984

Carani et al. 1990a

Gooren 1987

Morales et al. 1997

O'Carrolland Bancroft 1984

Skakkebaek etal. 1981

Sexual arousal

-

Anderson etal. 1992

Bancroft 1984

Carani et al. 1990a

Gooren 1987

Morales et al. 1997

Su et al. 1993

Spontaneous erections

Carani et al. 1992

Carani et al. 1990a

(during sleep, in the

Schiavi etal. 1988

Luisi and Franchi 1980

morning)

Salmimies etal. 1982

Ejaculation

Schiavi etal. 1988

Gooren 1987

Salmimies etal. 1982

Skakkebaek etal. 1981

Sexual activities with partner

Schiavi et al. 1988

Carani et al. 1990a

Orgasms in sexual activity

Knussmann etal. 1986

Davidson etal. 1979

(masturbation or coitus)

Mantzoros etal. 1995

Schiavi et al. 1988

Besides evidence from nonhuman primates and clinical case reports on effects of castration in human males (Nelson 1995), studies ofhypogonadal men on androgen replacement therapy provide convincing evidence of the essential role of androgens in some aspects of male sexual behaviour (Table 4.1). In patients with induced or spontaneous hypogonadism, pathological withdrawal as well as reintroduction of exogenous androgens affected the frequency of sexual phantasies, sexual arousal and desire, spontanenous erections during sleep and in the morning, ejaculation, sexual activities with and without a partner, and orgasms through coitus or masturbation (Bancroft 1984; 1986; Carani etal. 1990a; 1992; Davidson etal. 1979; Gooren 1987; Luisi and Franchi 1980; Morales et al. 1997; Salmimies et al. 1982; Schiavi et al. 1988; Skakkebaek etal. 1981).

There is only limited evidence on the effects of testosterone administration to eugonadal men with or without sexual problems. In a controlled study of eugo-nadal men with diminished sexual desire O'Carroll and Bancroft (1984) produced a significant increase in sexual interest with injections of testosterone esters when compared to placebo injections. But in most of the men studied the increase in sexual interest was not translated into an improvement of their sexual relationship -perhaps because psychological problems with their partner had not been resolved with hormonal treatment only. When supraphysiological doses of testosterone used as potential hormonal male contraceptive agents were administered to healthy volunteers, this resulted in a significant increase in psychosexual stimulation or arousal during testosterone substitution, although there was no change in sexual activity or spontaneous erections (Anderson etal. 1992;Bagatell etal. 1994; Su etal. 1993).

As the healthy male produces much higher levels of androgens than necessary to maintain sexual function, lowering serum testosterone levels to the normal low range or increasing them to the high normal range in eugonadal men has no appreciable effect on sexual function (Buena etal. 1993). This led to the conclusion that androgens are only beneficial in those men whose endogenous levels are abnormally low. However, Bancroft (1984) pointed out that we cannot be certain on this point because with increasing levels of endogenous androgen supply it becomes more difficult to manipulate the circulating levels with exogenous hormones. The homeostatic mechanisms are powerful and the more testosterone is administered, the more the individual's own supply is suppressed or the metabolic clearance rate is increased. In a study by Benkert et al. (1979), who gave eugonadal men testosterone undecanoate daily to treat erectile dysfunction, no increase in circulating hormone levels was achieved. Their failure to produce any behavioural effect on erectile function therefore may not be due to ineffective androgens, but rather a result of their failure to alter hormone levels.

Indeed, in several studies a significant relationship between physiological androgen levels and male sexual behaviour was observed. In a Swedish epidemiological investigation of 500 men aged 51 years low levels of non SHBG-bound testosterone were associated with low sexual interest (Nilsson et al. 1995). In young soldiers aged 18 to 22 years serum concentrations of 5a-dihydrotestosterone were a significant hormonal determinant of orgasmic frequency (Mantzoros et al. 1995). In young healthy volunteers Knussmann et al. (1986) could ascertain significantly positive correlations of salivary and total serum testosterone with the frequency of orgasms during the 48 hours following blood sampling. In their study, the majority of intraindividual correlation coefficients (from 6 samples per subject) were also positive but some negative and insignificant ones were found as well. This finding points to the great interindividual variability of behavioural responses to hormones, and it could explain contradictory results from other pertinent studies on testosterone levels and frequency of orgasms (Buena et al. 1993; Kraemer et al. 1976; Persky etal. 1978; Raboch and Starka 1972, 1973; Schwartz etal. 1980).

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