Women make testosterone in their ovaries and adrenal glands, which is then released into the blood stream. Some of that testosterone binds to a protein known as serum hormone binding globulin (SHBG) and this is the key: If testosterone is bound to SHBG, it is inactive. It is only free (unbound) testosterone that increases libido. Since birth control pills can increase the amount of SHBG, more testosterone will bind to it, which means less is left in the active form.
This does not occur in every woman, but studies show that those on hormonal contraception with decreased libido have lower free testosterone levels than women who do not take hormonal contraception. If that wasn’t bad enough, in addition to decreasing libido, low testosterone can lead to fatigue, lethargy and moodiness.