Women undergo a marked change in gonadal function with age, as evidenced by menopause, but the situation in men is less clear. Early studies demonstrated a well-defined change in total serum testosterone levels with age.187 However, because many of these studies involved men with systemic illnesses, and because these illnesses by themselves may significantly affect gonadal function, the results probably were not representative of the normal aging process without superimposed disease (see Chap. 199). Studies involving normal, older, healthy men have demonstrated few differences compared to younger men.188 Nevertheless, a small difference persists, especially when diurnal variations in total testosterone are examined.189 When the bioavailable (non–SHBG-bound) testosterone was measured, a more marked difference between young and elderly populations was discovered.190 Part of the reason for the greater difference in non–SHBG-bound testosterone than in total testosterone is the slight fall in total serum testosterone that occurs with age and the increase in SHBG, which further decreases the available testosterone.191 The reason for the increase in SHBG is not well understood. Explanations include stimulation by the increased serum estrogen that occurs with age, possible alteration in response to decreased thyroid hormones, or a change in glycosylation of the SHBG that might prolong the circulating half-life of this protein.

One aspect of male gonadal function that changes significantly with age is potency. Some investigators have interpreted this change in potency as indicating a change in gonadal function.192 When these men have been examined critically, however, their serum testosterone levels and other aspects of gonadal function have been found to be only minimally different from those of potent age-matched men, suggesting that hypogonadism alone is an uncommon cause of impotence in elderly men.193 The most common finding in this group is a high incidence of chronic systemic disease, which may involve the neurologic or vascular systems necessary for potency, or may necessitate the use of medications such as antihypertensive agents, which affect potency. In addition to an effect on Leydig cell function, age also appears to cause a decline in spermatogenesis.