Vasectomy has been identified as a possible risk factor for prostate cancer in seven case-control studies [34, 77-83] and two cohort studies [84, 85], but no elevation of risk was found in six other case control studies [86-91] and two retrospective cohort studies [92-94].Although a meta-analysis of 14 studies indicated that there is no causal relation between vasectomy and prostate cancer [95], further studies will be required to establish definitively whether vasectomy is a true risk factor for prostate cancer [96 - 98]. Three mechanisms by which vasectomy could enhance risk have been proposed: elevation of circulating androgens, immunologic mechanisms involving anti-sperm antibodies, and reduction of seminal fluid production [34,77,78,84,89,97,99]. Most studies of pi-tuitary-gonadal function did not find any effect of vasectomy [100-104], but some studies found changes in hormone levels [89,99,105-109]. There are reports in vasectomized men of slightly elevated circulating testosterone levels [34, 99, 107, 110], elevated levels of 5a-dihydrotestosterone (DHT), the active metabolite of testosterone in the prostate, a decrease in SHBG [89], and an increase in the ratio of testosterone to SHBG [34]. These reports suggest an elevation of circulating free testosterone following vasectomy which may be a critical factor associated with risk for prostate cancer, but specific mechanisms whereby vasectomy could influence the hypothalamo-pituitary-gonadal axis are not known.

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