Edward Blake, a 53-year-old forklift driver from Houston, sought out the same doctor for the same reasons. His testosterone measured 450, within the normal range. “I was feeling kind of tired and stuff, but after the third shot, I’m lifting things with no problem,” said Mr. Blake. He said in addition to feeling stronger, his sex drive improved.

Dr. Pastuszak said he primarily prescribes testosterone to men in the F.D.A.’s low category but will sometimes let other men with symptoms try it. “If you have these guys in the mid range and you put them on it, the majority will say they want to stay on it,” he said, adding that most men will say it makes them feel better and boosts their sex drive. But is that the power of suggestion or the power of the hormone?

“The reality is we don’t have the answer,” said Dr. Pastuszak, noting that there’s a big gray area. “I have to take their word that they feel better on it, whether that’s real or whether it’s placebo.”

Complicating matters, testosterone levels fluctuate, peaking around 8 a.m. and diminishing throughout the day. Levels tend to be lowest around 8 in the evening, then climb during the night. The peaks and valleys are larger for men 40 and younger compared to men in their 70s. (For a 40-year-old, a morning testosterone reading may be 200 points higher than in the evening, versus a 50-point difference for a 70-year-old.)

And all sorts of things can nudge levels in either direction.

Resistance training increases levels, as does a high-intensity workout. Even watching your favorite sports team win can nudge numbers up, as a 1998 study that measured testosterone among basketball fans before and after a game found. (Testosterone levels declined among those rooting for the losers.) Still, any gains from such activities tend to be fleeting; levels generally return to the individual’s normal within a half-hour or so.

And just as there are things men can do to boost levels, there are activities that lower testosterone scores. Endurance exercises, such as marathon training or cycling long distances, can lower levels, as can stress. Dr. Bhasin said that the kind of training endured by special armed forces — tough exercise, lack of sleep and food — can cause testosterone to drop to the levels of men who have been castrated — lower than 50.

Obesity causes testosterone levels to plummet — while losing 10 percent of body fat can increase levels by 100 points. Even taking care of the kids for several hours can cause levels to drop, a study in the Proceedings of the National Academy of Sciences reported. Flu and other viral illnesses can also cause levels to drop, so you should hold off testing until you’re fully recovered.