Low libido, or loss of interest in sex, has been a problem in relationships throughout the millennia of recorded history. Although loss of sex drive is generally associated with couple in mid-life and later, it is an increasing problem even for people in their 20's. Any loss of interest in lovemaking is likely to cause relationship issues, especially if sexual activity at one time was intense.

The hormonal "fuel" for sexual desire in both men and women is testosterone, the hormone that causes the outward expression of male gender markers but that is also found in lesser concentrations in women.

Sex Drive Killers in Men.

Men with intact testicles produce testosterone throughout life, but in declining amounts as they age. Most over 40 go through a stage of life sometimes termed "andropause." Every year after 40, a man's body produces about 1 per cent less testosterone. The reduction in testosterone reduces sperm production, muscle growth, and memory skills as well as sex drive.

Overweight men get a hormonal double whammy by virtue of the fact that fat cells produce estrogen, further reducing sex drive, and reducing one of the most powerful incentives to lose weight. Men who snore usually have lower levels of testosterone, and the hormone is also diminished by diabetes, cancer and chemotherapy, hyperthyroidism, hypothyroidism, liver disease, diabetes, hemochromatosis (iron overload disease), and many medications. Of all these conditions, the most common and most correctible cause of diminished sex drive is obesity.

Sex Drive Killers in Women.

Even at birth, a woman's body contains a small amount of testosterone, but it is only after puberty that the adrenal glands begin to make testosterone in amounts that influence sex drive.

About the time of a woman's first menstruation, the adrenals begin to make massive amounts of DHEA (dehydroepiandrosterone) from cholesterol. Some of the DHEA forms the stress hormones that are well known in the teenage years. Some of the DHEA forms estrogen and progesterone to prepare the uterus for conception and pregnancy. And a small amount of DHEA goes to the ovaries for conversion into testosterone to stimulate sexual desire. During the child-bearing years, a woman's sexual desire is highest in the middle of her menstrual cycle, when her body is also producing the high amount of progesterone (which makes the womb ready to receive a fertilized egg) and when she is mostly likely to ovulate.

Except in some relatively uncommon conditions like polycystic ovarian disease, a woman's body never makes a lot of testosterone, and production of testosterone peaks around age 30. Taking the Pill reduces production of testosterone and eliminates the monthly peaks of sexual interest. Production of testosterone, however, continues in the ovaries well after menopause.

In women, loss of libido can be associated with low levels of testosterone, but it is also influenced by low levels of estrogen. When the post-menopausal body stops making more than a minimal amount of estrogen, the vagina may become dry, making intercourse painful. Hysterectomy and oophorectomy (removal of the ovaries) further reduce hormone production. And, since women have longer life spans than men, heterosexual women often lose their familiar partners in intimacy even as they also have to deal with reduced hormone levels.