Your seemingly simple question requires some complicated explaining and then the final conclusion is, "It Depends." You see, there are two types of testosterone floating around in a guy's bloodstream and three ways of measuring it. Bound testosterone is hitching a ride on a protein molecule, such as albumin or globulin and is inactive; that is, it could spring into action at any time -- but not right now.
Free testosterone, on the other hand, is unattached and on the prowl. You've probably already guessed that there are far fewer of those free-spirited types than there are of the bound, a ratio of 5:95 or about 5% of total testosterone. But it's these few free 5% that drive the sex drive.
If a man (and an occasional woman) is concerned about a *really* sluggish sex drive he (or she) may request blood tests. However, a total testosterone level (bound and free combined) reading can be misleading. The bound and free levels need to be separated out to get the clearest picture of what is really going on. For even if the total levels of testosterone appear normal, or even high, if levels of free testosterone are below normal a quantifiable problem exists.
Then there is the matter of capturing those micro-critters--no easy task. The secretion of testosterone is regulated by LHRH secretions from the brain which in turn is affected by seasonal changes. This means that testosterone levels can fluctuate dramatically (up to 100 ng/dl) in as little as fifteen minutes.
Over the course of a day the levels can vary by as much as 50%. What this all boils down to is that a single blood test is unlikely to yield an accurate picture and that three drawings at fifteen to twenty minute intervals are called for. This is not a pleasant thought for the needle shy or faint-at-the sight-of-blood folks!
When you've gone through all that, you can finally get to some numbers. And these numbers are sometimes hard to decipher. The *normal* range for total testosterone is incredibly broad -- 250 to 1,200 nanograms (ng) per deciliter (dl). These numbers indicate whether there is adequate testosterone for normal sexual drive. If total level are in the "low normal" range, less than 400 ng/dl levels should be monitored. If levels are below normal, less than 250 ng/dl, your physician may recommend replacement therapy.
What about the free testosterone levels? Here comes the "It Depends." It depends on a man's age. The normal range of free testosterone is 1.0-5.0 ng/dl. However, a guy in his 20's may have levels around 41.0 pg/ml ( p grams/milliliter) while a gentleman in his 80's may have around 9.0 pg/ml -- and they would both be considered normal.
"Normal" and "not normal" can often be hard to define in any situation. Especially in the case of testosterone there are no simple answers.