New research shows older men's sex hormone levels depend on both race and geographical location, casting further doubt on the criteria for male menopause.

More than a million testosterone prescriptions are being written in the U.S. every year, experts say, and many go to middle-aged and older men with stunted libido and depressed mood presumably caused by low levels of the male sex hormone.

But there is no consensus about what these symptoms -- sometimes called male menopause or low T -- mean, or when testosterone levels are too low in the first place. (See Reuters Health story of June 16, 2010: Doubts cast on male menopause criteria.)

In the new study, published in the Journal of Clinical Endocrinology & Metabolism, researchers measured sex hormones in more than 5,000 elderly men from five countries across the globe.

They found average testosterone levels varied by 18 percent, landing twice as many white Americans (6 percent) in the low range compared with men from Hong Kong and Japan.

Low testosterone, also called hypogonadism, has been linked to erectile dysfunction and poor libido.

According to Solvay Pharmaceuticals, which makes a prescription ointment for men with low testosterone, the hormone is considered low when it dips below 300 nanograms per deciliter of blood.

However, skeptics say the symptoms of low testosterone may really just be the symptoms of old age, and there is no agreement about how low the hormone has to go before it becomes a problem. (See Reuters Health story of June 8, 2010: Do you have 'Low T?' Or is it just hype?)

International variation in sex (hormone) levels could have obvious implications for the diagnosis and treatment of hypogonadism, write the researchers on the new study, led by Dr. Eric Orwoll of the Oregon Health & Science University in Portland.

They say it is unclear why the hormone levels differ across the globe. At least part of the reason has to do with environmental factors such as diet, chemicals or social status, it appears, because Asian men who lived in the U.S. had testosterone levels similar to other Americans, while those who lived in Asia did not.

Testosterone wasn't the only hormone that varied. Black men in the U.S. and Tobago had higher levels of the female sex hormone estradiol, for instance, which has been tied to a lower risk of bone fractures.

These results may have important health implications, the researchers write.

SOURCE: Journal of Clinical Endocrinology & Metabolism, online June 28, 2010.