UPDATE 1-Testosterone gel linked to heart problems

* Older men with poor mobility studied

* Side effects spur early end to test

* Treatment improves strength, muscle mass

(Adds comment from company paragraphs 11-12,16)

By Gene Emery

BOSTON, June 30 (Reuters) - Testosterone treatments may build muscle mass in older men, but they may carry a risk of heart problems in people with poor mobility, U.S. researchers said on Wednesday.

The Massachusetts study, reported online by the New England Journal of Medicine, was halted after six months because the men using a hormone gel were developing so many heart, breathing and skin problems compared to patients applying a placebo gel to their shoulders or upper arms every day.

"I think the study raises important questions about the safety of giving testosterone to older individuals," Dr. Shalender Bhasin of the Boston University School of Medicine said in a telephone interview.

Levels of testosterone, the so-called male hormone, decline with age in men. Supplementing it in healthy men can build muscle mass and strength and lower the risk of disability.

The new test was the first to assess its effectiveness in men over 65 who already had mobility problems such as difficulty walking two blocks or climbing 10 stairs.

The 209 volunteers, with an average age of 74, also tended to suffer from obesity, diabetes, high blood pressure and high cholesterol at the start of the test. "One would expect that from a frail, older population," said Bhasin.

Recipients of the testosterone gel, sold under the brand name Testim by Auxilium Pharmaceuticals (AUXL.O), became better at walking up stairs and performing chest- and leg-press exercises.

But by the time the study was terminated, 23 patients in the testosterone group and five in the placebo group had experienced a bad side effect such as fainting, chest pain or heart attack. One man in the testosterone group died of a suspected heart attack.

The numbers were too small to be statistically significant and the side effects encompassed a lot of different diagnoses, which may mean that chance played a role in the results.

Yet men receiving testosterone had more serious side effects and more side effects considered to be life-threatening, the researchers said, lasting for three months after the trial ended.

"The study was designed to study mobility limitation, a common syndrome with the elderly that predicts disability, poor quality of life and mortality," Auxilium said in a statement.

"We believe these men are not representative of the typical testosterone replacement therapy population."

Bhasin was surprised by the finding.

"Testosterone is not currently approved for older men with age-related decline or mobility problems," he said. However it is approved by the U.S. Food and Drug Administration for other patients.

Doctors thinking of prescribing testosterone for their older patients should realize that the treatment may pose a serious risk, Bhasin said. "There may be safety issues that they should weigh in their decision," he said.

Testosterone causes salt and water retention and that could have been a factor, the researchers said. The National Institute on Aging, which paid for the trial, also noted that the men in the study may have been getting exceptionally high doses of testosterone. Auxilium said they were getting double to triple the recommended dose.

(Editing by Maggie Fox and Mohammed Zargham)