Population Council biomedical researchers are exploring the uses of hormone therapy for both men and women. The major reproductive hormones—estrogen and progesterone in women, testosterone in men—play important roles in regulating many normal body functions. Estrogen and progesterone are essential to fertility and successful pregnancy. Testosterone maintains muscle mass and libido.

As individuals age, the body’s production of the reproductive hormones declines. In women the decrease in hormone production is major and occurs abruptly when the ovarian function ceases at the time of menopause. In some but not all men there is a slow and gradual decline in hormone levels beginning in the mid fifties; hormone levels can drop below the lower levels of the normal range. When hormone levels decrease, women and men may experience a decrease in bone density accompanied by an increase in the risk of hip and spinal fractures. In susceptible individuals, reduced hormone secretion may also lead to hot flashes, sleep disruptions, loss of libido, and other undesirable effects on sexual function, as well as to changes in mood and energy level.

Hormone therapy for women who have undergone menopause may be accompanied by side effects, including an increased risk of certain medical conditions. Some of these side effects may be caused by the type of steroidal hormones used in the hormone therapy. Progestins are progesterone-like synthetic hormones. They are used in place of natural progesterone because until recently progesterone could be delivered only by injection. Although progesterone is now available orally, it must be given in much higher doses than progestins to achieve desired effects. Some progestins—for example, Nestorone^®, a progestin being investigated by Population Council scientists—do not have the testosterone-like effects of some older, widely used progestins. This may make them safer for use in hormone therapy.

Population Council researchers are also examining the potential of MENT^® (7a-methyl-19-nortestosterone) in treating the symptoms associated with a decline in the natural production of testosterone in men. MENT, a synthetic androgen (a testosterone-like hormone), does not stimulate the prostate as testosterone used in hormone therapy can.