In older men, decreased sexual activity and desire, not erectile dysfunction, may cause serum testosterone to decline, a new study from Australia finds. The results will be presented Saturday March 7, at ENDO 2015, the annual meeting of the Endocrine Society, in San Diego.

"We found that over two years, men with declining serum concentrations of testosterone were more likely to develop a significant decrease in their sexual activity and sexual desire. In older men, decreased sexual activity and desire may be a cause -- not an effect -- of low circulating testosterone level," said lead study author Benjumin Hsu, MPH, PhD candidate in the School of Public Health and the ANZAC Research Institute of the University of Sydney in New South Wales, Australia.

Whether decreasing sexual function is a cause or an effect of reduced androgen status in older men, or whether some other age-related factor may be involved, is not clear.

To explore the relationship between declining reproductive hormones and decreasing sexual function in older men, Hsu and his colleagues assessed men 70 years of age and above in Sydney, Australia, who took part in the Concord Health and Ageing in Men Project (CHAMP). The researchers tested the men at baseline (n=1,705) and again two years later (n=1,367).

At both visits, the participants answered questions about their sexual functions, including, "How often are you able to get and keep an erection that is firm enough for satisfactory sexual activity?"; "How many times over the last month have you had sexual activity (including intercourse and masturbation) reaching ejaculation?"; and, "How much desire for sex do you have now, compared with when you were 50?"

At both visits, the researchers also measured the men's serum testosterone, dihydrotestosterone (DHT), estradiol (E2), and estrone (E1) by liquid chromatography-tandem mass spectrometry; and they measured the men's sex hormone-binding globulin (SHBG), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) by immunoassay.

Over two years, baseline serum testosterone, DHT, E2 and E1 did not predict decline in sexual activity, sexual desire and erectile function. By contrast, the decline in testosterone (but not in DHT, E2 or E1) over time, though less than 10%, was strongly related to decreased sexual activity and desire, but not to erectile dysfunction.

The National Health and Medical Research Council, Sydney Medical School Foundation and Ageing and Alzheimer's Institute funded the CHAMP study.