Hormone producing organs are called endocrine organs. Primarily, women have three for the reproductive system working together (see figure at left). Hormones travel in tiny, potent quantities through the blood, interacting with receptor sites that coordinate the endocrine organs. This constant dance of hormones orchestrates our fertility, a dance that changes throughout the month and throughout our lives.

Of the millions of follicles (eggs) in the ovaries, eight to ten mature each month. But, in most cases, only one reaches the point that it ruptures and releases an egg (ovulation). The unused follicles die. After ovulation, the ruptured follicle reconnects and continues to grow, becoming the corpus luteum. In preparation for fertilization, it produces estrogen and progesterone to create an extra lining in the uterus. But after about 12 days, if no fertilization occurs, the corpus luteum dies, and estrogen and progesterone levels plunge. This causes the lining to be expelled. Menstruation begins.

Hormonally, a woman's period marks the end of the menstrual cycle. A new cycle begins when the hypothalamus, in response to the dramatic drop in estrogen and progesterone levels at the end of the last cycle, sends GnRh (another hormone) to the pituitary, which then releases a new batch of follicle stimulating hormones (FSH) to the ovaries. This stimulates the maturation of a small group of follicles. At mid-cycle, a critical peak of estrogen production alerts the hypothalamus to create more GnRh. This time it tells the pituitary to release its lutenizing hormone (LH) to the ovaries, a trigger that will spur the small explosion that releases the egg from the dominant follicle. Ovulation occurs and the corpus luteum forms to continue estrogen and progesterone production until it dies in the absence of pregnancy. Hormone levels plunge. In perimenopause, the ovaries' estrogen and progesterone production shifts from one that's follicle rich (with an emphasis on creating eggs) to one that's stromal rich (less emphasis on eggs, more on the stroma that holds the eggs together). As a result, the hormonal dance changes. A follicle releases an egg less and less often, so estrogen and progesterone levels drop. The uterus lining ceases to thicken, so there is little or nothing to discharge. Menstruation stops, and the decline of ovarian hormones sparks elevation of pituitary hormones such as the luteinizing hormone. It is suspected that this rise of LH in the pituitary causes hot flashes. In addition, these dramatic changes to the endocrine organs are associated with various other common signs of menopause such as headaches, sleeplessness and mood swings.*