Final answer:
Estrogen is the hormone that when too low can lead to disturbances in the reproductive cycles of female athletes, affecting menstrual regularity and bone density.
Step-by-step explanation:
The hormone that, if too low, can cause disturbances in the reproductive cycles of female athletes is estrogen. This hormone plays a critical role in maintaining regular reproductive cycles. Low estrogen levels can lead to decreased bone density and disruptions in the menstrual cycle, adversely affecting overall reproductive health.
Estrogen is responsible for the secondary sexual characteristics of females, endometrial regrowth, ovulation, and calcium absorption. Female athletes often suffer from conditions such as amenorrhea or oligomenorrhea, which are characterized by absent or infrequent menstruation, often due to low body fat and high physical stress that reduce estrogen levels.
Progesterone also plays a significant role in the female reproductive cycle, but the hormone primarily associated with menstrual irregularities due to athletic stress is estrogen. On the other hand, cortisol is a stress hormone, and testosterone is primarily a male sex hormone, although it is also present in females in lower amounts.