Lifestyle
Being very underweight, severely obese, or malnourished can cause a low libido due to disruptions in normal hormonal levels. There is also evidence to support that specific foods have an effect on libido.
Under Medications
Reduced libido is also often iatrogenic and can be caused by many medications, such as hormonal contraception, SSRIs and other antidepressants, antipsychotics, opioids and beta blockers. In some cases iatrogenic impotence or other sexual dysfunction can be permanent, as in PSSD.
Testosterone is one of the hormones controlling libido in human beings. Emerging research is showing that hormonal contraception methods like "the pill" (which rely on estrogen and progesterone together) are causing low libido in females by elevating levels of Sex hormone binding globulin (SHBG). SHBG binds to sex hormones, including testosterone, rendering them unavailable. Research is showing that even after ending a hormonal contraceptive method, SHBG levels remain elevated and no reliable data exists to predict when this phenomenon will diminish. Some question whether "the pill" and other hormonal methods (Depo-Provera, Norplant, etc) have permanently altered gene expression by epigenetic mechanisms.
Left untreated, with low in testosterone levels will experience loss of libido which in turn can often cause relationship stress, and loss of bone and muscle mass throughout their lives. (Low testosterone may also be responsible for certain kinds of depression and low energy states.)
Conversely, increased androgen steroids (e.g. testosterone) generally have a positive correlation with libido in both sexes.
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Psychological and Physical Factors