I have this as well, and as you noted it is low testosterone. It can be caused by any number of things, but basically the body does not produce enough testosterone to be considered in the "normal" range, which varies somewhat depending on the source you consult but is generally 300 - 1,100.
In my own case, I have a small microadenoma (tumor) on my pituitary gland that is inhibiting it from secreting Leutenizing Hormone (LH). The LH triggers the body to produce testosterone, so since the LH is low, then my testosterone is low. My ratings have been consistently about 200 - 270 (comparable to an 80 year old man, since our bodies produce less and less testosterone as we age) prior to starting a regimen of hormone replacement therapy (HRT). I use a daily application of a gel-based substance called Androgel that is an alcohol base so the gel evaporates after application, leaving the testosterone on the skin to be absorbed through the skin throughout the day. It's rubbed on the sholders and abdomen. My ratings are generally about 400 which is normal for my age (47). During our teen years, the ratings would be about 1,000 or more, hence the term "raging hormones" in teens.
The symptoms in me that something was wrong and sent me to a doctor 5 years ago were tiredness (testosterone gives us strength), emotional issues (testosterone helps regulate our emotions), erection issues (testosterone helps with libido), and a general feeling of malaise. Having been on HRT for several years now, I can definately tell when I have been off it for a week or so. It's made a world of difference.
Below is a really good Wiki article about hypogonadism:http://en.wikipedia.org/wiki/Hypogonadism