Premenstrual syndrome may be the result of a cyclic mood disorder that is synchronized with, but not caused by, the menstrual cycle, a study suggests.

The study, in the New England Journal of Medicine, says PMS also may be triggered by as-yet unidentified changes related to reproduction and hormones, not just hormone level changes that occur during a woman's premenstrual phase.The new research helps explain why using hormone therapy is ineffective in treating PMS.

The study was conducted by researchers at the National Institute of Mental Health and the National Institute of Child Health and Development in Bethesda, Md.

An estimated 5 percent of American women are affected by PMS and experience behavioral, emotional and physical symptoms severe enough to affect their lives.

Symptoms include sadness, irritability, anger, tension, loss of energy and changes in appetite and sleep.

The new findings match previous studies showing that therapy with the hormone progesterone is ineffective. Although progesterone is not approved by the Food and Drug Administration for treatment of PMS, many doctors prescribe it.