The study is the largest placebo-controlled trial in older men conducted on the subject to date. The sexual function study is part of the Testosterone Trials, a series of seven studies examining the effectiveness of hormone therapy in men who are 65 or older, who have low testosterone levels and are experiencing symptoms of testosterone deficiency. The research is supported primarily by the National Institutes of Health.

Testosterone is a key male sex hormone involved in maintaining sex drive, erectile function and sperm production. The Endocrine Society’s Clinical Practice Guideline recommends using testosterone therapy to treat men with symptoms of androgen deficiency and low levels of testosterone. Androgen deficiency occurs when a man has consistently low levels of testosterone and resulting symptoms such as sexual dysfunction.

In the past 15 years, use of testosterone therapy has rapidly expanded among men. Testosterone levels decline as men age, and some men develop low testosterone levels and symptoms. Since 2000, the number of men beginning testosterone therapy has almost quadrupled in the United States, according to a 2014 study published in The Journal of Clinical Endocrinology & Metabolism.

“Our findings indicate low testosterone is one cause contributing to reduced libido and erectile dysfunction in older men,” said the study’s first author, Glenn R. Cunningham, MD, of Baylor College of Medicine and Baylor St. Luke’s Medical Center in Houston, TX. “Men experiencing these symptoms should be evaluated for testosterone deficiency.”

The study was designed to investigate the effectiveness of testosterone therapy. It was not large enough or long enough to address issues related to cardiovascular events or clinical prostate cancer.

The placebo-controlled, double-blinded trial examined the effect of testosterone therapy on sexual function in a group of 470 men. The men were enrolled in the study through 12 academic medical centers. The participants were at least 65 years old and had low testosterone levels, based on the average results of multiple tests. All of the men had a heterosexual partner.

During the year-long study, the men were assigned to receive either testosterone gel or a placebo applied to the skin. The participants answered questionnaires about sexual function at the outset and every three months during the 12-month study.

The men treated with testosterone therapy displayed consistent improvement in libido and in 10 of the 12 sexual activity measurements, including frequency of intercourse, masturbation and nighttime erections. In comparison, men who received the placebo did not change their questionnaire responses significantly over the course of the year-long study.

“For symptomatic older men with low testosterone levels, testosterone therapy led to consistent improvement in most types of sexual activity,” Cunningham said.