Androgens in Health and Disease.pdf - E Library

Chapter 14/Androgens and Sexual Function 263
Normal Variations in Androgen Levels
Although the normal range of plasma T is large, there is little predictable relationship
between T levels within this range and measures of sexual interest or behavior (24–26).
Two studies in which erectile response to erotic stimuli was measured in the laboratory
found a correlation between plasma T level and latency, but not degree of erectile
response (27,28). It is generally assumed that there is a level of plasma T above which
increases in T will have little effect. This level is presumably within the normal range,
may vary across individuals, and, as discussed earlier, could well increase with age. If
such assumptions are correct, then the lack of correlations between circulating T and
sexual parameters is not surprising, at least for younger men.
Testosterone shows a clear circadian pattern in younger men, with levels being at their
highest during the latter part of the night and early morning and lowest in the evening.
This pattern flattens out in older men (29). The timing of this rhythm does vary somewhat
on a seasonal basis, but the very limited evidence (30) suggests no relationship between
the circadian rhythm and fluctuations in sexual interest or activity, but a possible relationship
between the seasonal variation and seasonal peaks of sexual activity (30–32).
LOWERING TESTOSTERONE
Experimental Manipulations in Normal Men
In a study of normal men, aged 20–40, Bagatell et al. (33) compared the effects of
lowering T by means of a GnRH antagonist, NalGlu, on its own and in combination with
different doses of T replacement. The design included a placebo group. This experimental
manipulation lasted 6 wk, with 4-wk “pretreatment” and “posttreatment” phases. The
medications were given on a weekly basis. The results clearly showed the reduction of
sexual interest and activity when T levels were reduced with the NalGlu alone, consistent
with the pattern of behavioral effects described earlier in the studies of hypogonadal
men. In the lowest T-replacement regime (50 mg T enanthate weekly), the adverse
behavioral changes were avoided even though blood levels of T were substantially lower
than those pretreatment. However, blood samples were taken immediately before each
weekly dose and, therefore, the T levels reported would be lower than those in the earlier
part of the week. It is distinctly possible that if those end-of-week levels had been
sustained for longer periods, a more negative picture would have appeared.
INCREASING TESTOSTERONE
The effects of increasing T in eugonadal men has become a topical issue because of
the development of male contraceptives involving T administration. Anderson et al. (34)
assessed the effects of supraphysiological levels of T in a single-blind, placebo-controlled
study involving 31 healthy men who were part of an ongoing efficacy trial of
hormonal male contraception. T enanthate injections 200 mg weekly were given for 8
wk in one group (T-only group), and placebo injections weekly were given for 4 wk,
followed by 4 wk of the T enanthate injections in the other group (placebo/T group).
Neither group showed any change in measures of sexual interactions with the partner or
in frequency of masturbation. In both groups, however, there was an increase in scores
on the Subscale 2 of the Sexual Experience Scale (35) during T administration, but not
during placebo. This subscale measures the extent to which an individual seeks or allows
(rather than avoids or rejects) sexual stimuli of an audiovisual or imaginary kind. It is,
therefore, an index of sexual interest independent of interaction with a partner.