THURSDAY, Sept. 13 (HealthDay News) -- New research suggests that one out of four men over 30 have low testosterone levels, but only one out of every 20 men have clinical symptoms linked to such a deficiency.

As men age, they are more likely to experience symptoms such as lack of sex drive and erectile dysfunction as a result of low levels of testosterone, the researchers explain.

Low testosterone is defined as 300ng/dL (nanograms per deciliter) of total testosterone and less than 5 ng/dL of free testosterone. Free testosterone is the amount of hormone not bound to other proteins.

Low testosterone can indicate androgen deficiency if it is accompanied by low sex drive, erectile dysfunction, osteoporosis or fracture, and two or more of the following: sleep disturbance, depressed mood, lethargy or diminished physical performance.

"Low levels of testosterone impact many aspects of male physiology," lead author Andre B. Araujo, a research scientist at the New England Research Institutes in Watertown, Mass., said in a prepared statement. "This is particularly significant because the ongoing aging of the U.S. male population is likely to cause the number of men suffering from androgen deficiency to increase appreciably."

New England Research Institute scientists analyzed data from almost 1,500 men enrolled in the Boston Area Community Health Survey. The survey tracks people aged 30 to 79 years and compiles data on factors such as testosterone, symptoms of hormone deficiency, and medications that may impact sex hormone levels.

Approximately 24 percent of the men surveyed had low total testosterone and 11 percent had low levels of free testosterone. Many of the men had no symptoms related to their low testosterone.

About 5.6 percent of the men in this study suffered from symptomatic androgen deficiency. Older men were especially prone: over 18 percent of men over age 70 met the criteria for this deficiency.

Based on these results, the researchers predict that by 2025 there may be as many as 6.5 million American men between 30 and 79 years of age with symptomatic androgen deficiency, an increase of 38 percent from the year 2000 population estimates.

"This study did not assess whether men with symptomatic androgen deficiency are good candidates for testosterone therapy," said Araujo. "Well-designed, randomized, placebo-controlled trials would be needed to address the risks and benefits of testosterone therapy."

The study is available online and will appear in the November issue of the Journal of Clinical Endocrinology & Metabolism.

More information

To learn about androgen deficiency in men, visit The Men's Health Network.

SOURCE: Endocrine Society, news release, Sept. 6, 2007