If you're a man and you're concerned about low levels of testosterone, doctors say there are a key steps to take before you go with testosterone supplementation.

First off, have your testosterone level measured. It's a simple blood test that should be done first thing in the morning, according to Dr. Ronald Tamler, an endocrinologist and clinical director of the Mount Sinai Diabetes Center.

"More than other hormones, testosterone goes up and down throughout the course of the day, and it's highest between 8 and 10 in the morning," he says. "What this means is you can give someone a 'false' low testosterone level if you measure it in the afternoon."

So the test should be done between 8 and 10 am, Tamler says. And it needs to be done twice, on different days, to confirm results.

If levels are low, Tamler says there are a number of reasons why that could be. "The most common reason for low testosterone in my practice, I find, is obesity," he says. That's because fat tissue, especially abdominal fat, can turn testosterone into the female hormone estradiol.

That transformation has two effects, Tamler says. First, there's just less testosterone in the body. And second, the extra estradiol disrupts signals to the brain that stimulate testosterone production, telling the body not to produce more testosterone.

When obese men had weight loss surgery and lost about 40 pounds, their testosterone levels doubled in just one year, Tamler found in a recent study.

Diabetes and sleep apnea can also cause low testosterone and should be treated first, he says, before going on hormone therapy.

If it turns out that patients have none of these problems and their testosterone levels are truly low, then doses of the supplement need to be carefully tweaked in order to make sure it doesn't cause production of too many red blood cells, which can lead to deadly blood clots.

"The amount of red blood cells in circulation can go up very high, and that can mean the blood does not flow as smoothly as it should and that can give you troubles," Tamler says. Clots can cause heart attacks and strokes.

People on testosterone therapy need to be re-evaluated every six months, Tamler says, to check on red blood cell production, blood pressure and estradiol.

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