Women who perform considerable amounts of exercise on a regular basis or lose a significant amount of weight are at risk of developing hypothalamic (or 'athletic') amenorrhoea. Functional Hypothalamic Amenorrhoea (FHA) can be caused by stress, weight loss, and/or excessive exercise. Many women who diet or who exercise at a high level do not take in enough calories to expend on their exercise as well as to maintain their normal menstrual cycles. The threshold of developing amenorrhoea appears to be dependent on low energy availability rather than absolute weight because a critical minimum amount of stored, easily mobilized energy is necessary to maintain regular menstrual cycles.