Actually, the mystery of low libido cannot always be resolved with an increase in testosterone. If only the solution was that easy for everyone. Brain chemistry and wiring also plays a role (as do emotional memories), and sexual medicine researchers are hard at work in their search for this holy grail.
If you google Leah Millheiser, MD, the Director of the Female Sexual Medicine program at Stanford University, you can read about her ongoing research in this area. I feel very privileged that she was able to make room in her busy practice for me.
Libido in both men and women is related closely to the level of testosterone in the blood (yes women have testosterone produced largely by the adrenal glands and men have so-called female hormones in their system). Testosterone level in the blood can be increased by a number of medications (androgel applied to the skin of the stomach, shots in the buttocks, etc.) all of which require a doctor's prescription. Too much and hair grows on your chest and your upper lip, falls out of your scalp and your voice gets deeper so be careful.