A small experiment has concluded that twice-daily injections of the hormone leptin, best known to regulate appetite and weight, can jump-start an idling reproductive system.

Doctors found that the treatment restored menstruation in five of eight female athletes who had become so lean they had stopped having their periods. Leptin is a naturally occurring hormone produced by fat cells, so athletes, dancers and other thin people may have less of it. Leptin is also being studied for its role in obesity.

But the researchers did not give placebo shots to the other six volunteers, so a more thorough two-year test of 80 women is poised to begin to see if the Amgen version of leptin, used in the pilot study, really works.

Christos Mantzoros of Beth Israel Deaconess Medical Center in Boston said leptin has the potential of helping women with the eating disorder anorexia nervosa, who make up 1 percent to 2 percent of college-age women; physically active women who have stopped menstruating (4 percent to 5 percent of the same age group), and women who are simply very thin but don’t realize they have a problem until they can’t conceive.

Perhaps one-third of women who later go to infertility clinics are not able to get pregnant because their leptin levels are low, Mantzoros said.

The study, which Amgen helped finance, appears in Thursday’s New England Journal of Medicine.