By Anne Harding

NEW YORK (Reuters Health) - Reduced sexual activity could cause a dip in testosterone levels in older men, new findings suggest.

Among men 70 and older, those who reported a decline in sexual activity and desire over a two-year period also showed small declines in serum testosterone, Dr. David Handelsman of the ANZAC Research Institute at the University of Sydney and Concord Hospital in New South Wales, Australia, and colleagues found.

They report their findings in the Journal of Clinical Endocrinology and Metabolism, online January 28.

"Decline in serum testosterone is more likely to be the result rather than the cause of sexual dysfunction among older men who don't have reproductive disorders," Dr. Handelsman told Reuters Health by email. "The widely prevalent misinterpretation of this (as if the mild lowering of serum testosterone needs or might benefit from testosterone treatment) is one of the main drivers of the massive over-use of testosterone prescriptions in North America over the last decade."

While declines in androgens and sexual function are both thought to be aging-related, Dr. Handelsman and his colleagues note in their report, the relationship between androgen levels and sexual function is not clear.

To better understand the temporal and predictive relationship between androgen levels and sexual function, the researchers looked at 1,226 men participating in the Concord Health and Ageing in Men Project (CHAMP), measuring their levels of testosterone, dihydrotestosterone, estradiol, and estrone with liquid chromatography-tandem mass spectrometry. Men also reported on their sexual function using standardized questions, at baseline and two years later.

Baseline hormone levels did not predict sexual function decline. However, there was a strong relationship between a decline in testosterone over the study period and a decline in sexual activity and desire.

For every one standard deviation drop in testosterone from baseline to follow-up, the adjusted odds ratio of further decline in sexual activity was 1.23. The decline in testosterone was "strikingly small," the researchers note, at less than 10%. Declines in estrogen were also associated with declines in sexual activity.

The changes in testosterone "are too small to explain the decrease in sexual activity or desire as well designed placebo clinical trials show that reducing testosterone by 75% or 80% is required to reduce male sexual function whereas reductions of 50% have no impact on sexual function in young...or older men," the researchers write.

Based on the findings, the best approach to treating sexual dysfunction in older men without reproductive disorders would be to "focus on correcting as far as possible underlying disorders that cause the sexual dysfunction as well as the mild lowering of serum testosterone which results from sexual inactivity," Dr. Handelsman said. "In any case testosterone treatment is not justifiable without convincing evidence from well conducted placebo controlled clinical trials which do not yet exist."